This was briefly a problem when my hubby and I started our training Fellowship. We solved it by allowing players to join directly; no invitation needed. The good players stayed, the bad left, and the inactive were weeded out. We now have a full Fellowship of eager, active players and I've now had to change my signature line from "join us" to a "let me know if you want to go on the waiting list".
This won't work for every Fellowship, but for our newbie training group, it did just fine. Yes, we still have a few players who don't participate as much as our occasionally over eager ones but that's ok with us.