"X Farm Group":

[peaches]

They're using "farm" to refer to the ability of the group, i.e. "have X on farm".

In other words, they just want geared and experienced people. That's the new fashionable term these days.

This is it exactly. They see that the people are (hopefully) experienced enough to farm it and just need one or two dps to fill the ranks. They join for a quick kill and leave. Best thing to do would be to talk to every person and make sure all of them want to farm it before queuing.