Early on, it didn't seem like these two knew each other very well. Over time, though, the image the show projects has shifted. Which take do you like? And you can talk about these two's histories in general here as well.

Fluttershy's background and relationship with RD is one of the things where I can work with the eventual canon we got or what I put together beforehand.

In my original model, they only knew each other briefly in summer flight camp and quite possibly wouldn't even have remembered each other if not for the role they played in getting each other their cutie marks. Them meeting up again in Ponyville was sheer coincidence, and the resultant tenuous friendship largely based around shared cultural background - ex-pats tend to bond regardless of whether they'd have given each other the time of day back home.

Over time however the show and other media have definitely leaned into a "childhood friends" interpretation, with Flutter Brutter more or less confirming it. (Interestingly, I don't think we even saw Fluttershy in Parental Glideance - what's that dynamic like?) So when Fluttershy has her canon family, I move the coincidental re-meeting to Rainbow's first year of high school (Cloudsdale's one of those places with a bylaw). Having a freshmare sticking up for her didn't really help Fluttershy socially, but her parents were just glad that she'd finally made a friend (Zephyr was glad that said friend was a hot mare less than a year older than him). They stayed in touch after Fluttershy graduated and by the time she finished college Rainbow was already in Ponyville and happened to mention that there was an open animal management job . A different route, but the destination is the same...

As for the first part of the question, obviously the above version doesn't allow for her moving to the ground, and I don't actually see her doing so in either. Rather, her parents/guardians used the lure of the ground as motivation for her to acquire basic flight competency, at which point she was allowed to spend most of her free time on the surface. (This may have contributed to her lack of friends.) She didn't actually make her home on the ground until her college years.

I always figure Fluttershy was eventually rescued, but went back to the ground the first chance she was able. She was, in fact, probably a outlier in terms of gaining independence, doing it at a much younger age unless she had some relative on the ground.

After that event, though, she definitely would have drifted apart from Dash, who was too busy focusing on flying to care much about the ground. It wouldn't be until she got the job doing weather in Ponyville that they reconnected. (Alternately, she remembered she had a friend in Ponyville and sought her out, to try and find out if she could score a job somewhere easy. They lived together for a while until she could build her cloud house.)

7038277
So how does Dash know her family?

7016950 Given what we saw in "Dragonshy" and even to an extent in "Hurricane Fluttershy" I would say that Fluttershy probably did grow apart from Rainbow Dash. And the bullying she endured at flight camp over her wings probably discouraged her from staying in Cloudsdale permanently.

19. Did Fluttershy remain on the ground after getting her cutie mark? Were she and Rainbow Dash friends all along, or did they drift apart for a while?
The flight camp instructors would eventually have found her and returned her to the camp. I imagine she moved groundside as soon as she could - probably within a year.

I'm not sure Fluttershy and Rainbow were all that close at flight camp, but by the time Twilight came to Ponyville they had definitely drifted apart.

Post-2014 addendum: This is one the show retconned. Dash and Flutter's don't seem all that close in season one but later the show decided they were close friends all along.

Fluttershy ultimately decided to live near Ponyville because she couldn't stop thinking about the animals, but I do believe that she did fly back up to Cloudsdale to finish out summer camp and to live a bit longer with her parents. Not only are Rainbow and Fluttershy friends, they are lovers; Rainbow Dash stayed in Ponyville because Fluttershy moved there. If RD had not fallen for FS, she would have remained in Cloudsdale where she had a better chance of getting noticed by the Wonderbolts. She lacks all things tact, but in her mind, her loyalty to Fluttershy is how she shows her love.

Not only are Rainbow and Fluttershy friends, they are lovers

A bold claim considering your general focus on the early series, where these two had far less close a relationship.

In my mind, they were friends who drifted apart for a time. RD got the job as a weathermare of Ponyville to stay around Fluttershy as they were good friends. However, over time RD got more and more into other things and their friendship drifted apart as a result. Fluttershy's lack of getting out and willingness to do things also contributed. RD would still visit from time to time but it was becoming less and less frequent. By the time of the first episode their friendship had more fallen to aquaintences.

As for Fluttershy remaining on the ground? As soon as she could, yes.

Well, Fluttershy was like eight when she fell on the ground so of course she didn't stay down there.

Also, Fluttershy and Dash became "feather sisters" each one has a feather from the other tucked inside their wing. While they were childhood friends, they did drift apart after high school, reconnecting in Ponyville though they had rough patches.

Hm. Yeah, basically what the others said. She was way too young to just 'stay down there', but this experience had opened her eyes and she knew she wanted to return. After all, she had made some friends. She maybe even convinced herself that they actually, truly needed her. Which is why she returned as soon as she could. Her parents aren't exactly rich, but they were always very supportive. They clumsily scoured the area for an affordable piece of land with a house on it, preferably. What later became her cottage was within budget. Fluttershy became independent way earlier than many others do. Funny thing: Since this was 'her house' now, there was no need to pay rent. And with her critter friends, there was rarely any need to pay for food, either. She could help them scavenge as much as they could help her. There was a time where she just... didn't need bits. At all.

Eventually, some critter fell ill and a warm blanket just didn't cut it. That way, she met the veterinarian. And once more, her eyes were opened. She learned as much as she could and eventually became somewhat of a veterinarian herself. Without official training and title (because thinking about school and students and teachers and exams results in anxiety), but well, does that really matter if the quality of the treatment is the same?

As for Rainbow. They definitely were childhood friends. But Rainbow never quite understood her fascination with the ground. It's just bugs and bears, who cares? She stayed up in Cloudsdale and focused on her training. The Shy's were still quite friendly with her, she was always welcome there, and whenever she really missed her friend, she took them up on that offer and swooped in. Much to Zephyr's delight. In a way, Rainbow might actually have spent more time with him than his own sister did.

Eventually, Dash wanted out. Her parents were very, very supportive. A tad too much, even. She could've stayed with them until her teeth fell out and her hair grayed. But she felt like choking on all that support. She regrets to this very day having accepted her parents' money. She wanted a house of her own. And they bought her a freaking PALACE. Welp. Nothing she could do about it now. And it is a cool house. One might even call it... awesome.

Thing is, though. While the house is nice and all, food doesn't just poof into existence. She needed bits, and she was not going to ask her parents for anything ever again. Sweet Celestia, who knows what crazy ideas they would come up with next! So she got herself a job. Asked at the weather factory, but they didn't have a spot free. However, they work closely together with the weather teams all over Equestria. Funny enough, there's a free spot nearby. Rural town, relatively quiet. She doesn't care much about the ground, buuut - that sounds perfect. Small town means less work to do. So more free time for training. And napping. (That's an art form.) Enough bits for food. Enough distance that her parents won't bother her too much. But close enough to visit them whenever she wants to, if she wants to. And the job itself even helps in her training. It's perfect!

It took a surprisingly long time for them - Fluttershy and Rainbow - to meet again. They lived in the same town without knowing for quite some time. But eventually, they reconnected.

7689094
An interesting and plausible approach.

(So you hold they were friends even before flight camp?)

7689162
Thank you. I don't know, somehow, I prefer the less tear-jerking, less catastrophic, less dramatic approaches. More grounded.

(So you hold they were friends even before flight camp?)

I think so, yes. (Don't quote me on it, though.)

7689183
A fair taste. (Though I think my take here is one of my "plainer" ones, too.)