Do the group leaders have power to change member groups, for their group only? I believea a code can do it. SMF should support it by default.
Group leaders don't have the ability to change member groups for their own group. I think this used to be the case however this ability was removed at one point or another most likely because of abuse. Staff members do the adding/removing of members from groups, some staff belonging to groups themselves.
I think, you could have a limit set on the Groups that could be edited by a user. It will be a small par** of a code put into the theme. Anyway, can Global Mods change usergroups?