Looks like a classic case where the share value will take a real hit.
Presumably the group has no agreed (written ) rules?
If so then its doen to all of you to agree the new shape of things. I'm guessing the club will no longer provide parking or maintenance so your costs will be higher. With no engine fund, you'll have to start building one.
Whilst the initial deal looks to have been pretty good financially it looks like you're now going to have to pay for it - either in direct costs or a reduced value of the share if you sell.
But if they are no rules to ddetermine what happens with shares then it down to being nice to everyone and making the best of it. I can see you're not keen on the increase in costs but I cann't see any reason why the club should n't do this.
Read the many threads on group aircraft to illustrate just how bad things can be....