What has worked best for you so far in attracing Members to / getting activity in your Group? 

I think playing games really helped- people started checking out the group more after I did that, because it was something amusing to pass their time at work. And while they were in the group for the game, they would post on other threads

I don't know how much it helps, but I've been sending a welcome toast to each new member, that way, if someone is browsing their profile, they'll see it.

that's a good idea Brianne!