I'm in a group where 3/5 people haven't logged in since release day. I know how this feels. I think it would be great to allow a group to kick, or allow players to leave group, especially if its a matter of more than half of your group just not playing. I've dumped a bunch of hours into gim, but it feels like regular ironman except the fact that there is one other person helping.
Warning players that their decision is final is not going to help this gamemode be healthy, I mean yeah, we can just let all the weaker/inactive groups die out and eventually even the active guys will say "f-it" because they can't really compete and they have nowhere to go.
..Or a counter-measure can be put into place so that the experience isn't ruined for a whole group because of 1,2,3 people.
It's just not very realistic that you'll have the same 5 guys being active 1 month, 2 months, 6 months down the line, even if you "know" them