Ah, yes, the dreaded /in/active groups. I've been in quite a few m'self to know that 'tis tough trusting any recruiters with their claims. I also know, from lots of experience, that some groups just don't last due to their chosen theme- and of course the hypocritical members that say they enjoy the group and will "be back tomorrow." Hah!
But yes, y'er members and y'er group theme are key to staying active. I once held a tribal wolf roleplay many moons ago that didn't last due to lack of members. I also held a tsavo lion pride (what a joke that was). 'Twas going to be highly realistic with lion traits and physical features, but no one wanted to join because the males can't grow manes. XD
So, what truly makes an active group and how do ye find one? Look f'er the group with active leaders (a lot of great roleplays 'ave been mentioned, so ye can tell their work has payed off) and active members- people ye see on with the leader a lot. Of course, the latter may be more difficult if their privately mapped (wee floofs hiding amongst the mappers, but I'm sure he'll find what y'er looking f'er!