I don't know how. Do you?

Usually people join, either by finding the group from searching or by seeing that stories they've read are a part of it.

Letting people in the Sci-Fi group know is probably the best you could do for now, well done. I'm not sure what other groups would be as interested. Star Trek perhaps?

3839841 I hate the waiting game.

3840594 Would advertising in a different group like that be considered rude, or am I being overly cautious?

3841996
I'm honestly not sure, but I suspect that different people and groups have different attitudes towards it. The main point to consider is how closely the group is related to B5, how likely people there are to be interested in it as well. Given the story of developing DS9, there's clearly a connection for Star Trek, although I've no idea if there's much, I dunno, denial and/or animosity regarding that within the Start Trek fandom.

3842269 I just made a promotional thread that should come across as polite rather than forceful or competitive. I guess we'll see how it goes!