The people saying "if they get better gear, they are helping the guild" are so full of shit. That kind of player is only in for itself and are in no way concerned with the progression of the guild. They'll advance as far as they can where they are and then leave first chance they see to better themselves.
Let them do their runs, but insist/ask nicely they give everybody interested in the guild the chance to join. If they ignore that, get rid of the toxic apples.
But, on the other side of things, if you've got really underperforming people (like dps doing 7k or so) you need to deal with that as well.