Moving mass members from one group to another

Okay, this isnt for PunBB, but Ill explain it like it is.

I have some members in one group, and I want to have a checklist so I can check the members I want to move over to another group.
The problem is that its not that simple. The number of members moving could be anywhere from 1-999999999999999. And there is a certain amount of members that can be moved at max (Depending on how many members are in the other group, and how many members that group can hold)

So I can make the checklist and all, but im stuck at when its submit. I think a foreach would work for this, but the submit button is part of the form, and I dont know how to use foreach.

So I need to check and see how many members the group can hold, comapre that to how many your moving, and somehow move all of the checked users.

If someone could post a bit of code, I would really appreciate it