My point is that by flat locking groups to a 'number of players' it doesn't, in fact, encourage players to recruit people of the appropriate level: It does the opposite, encourages them to recruit the most powerful person to fit in that slot.

What we need is something to make the at-level character somehow a reasonable swap with an over-leveled character.

I'd suggest that we limit the level of abilities used in dungeons either as a pastiche of abilities or a limiting effect. Just like when you're leveling up a lower level combat ability, but have the dungeons or content do it for your. So you enter a level 30 dungeon, and you're limited to level 30 or below abilities. It could downgrade the buttons automatically to their lower level. That way you don't really have a big benefit of having an over-leveled character in a dungeon.

Some gear abilities would also have to downgrade, but since everything has higher and lower levels, that is plausible...