I feel like they are a neat idea, but feel like giving them as anything other than random loot might feel like its OP, that being said I also believe making a set for any of the members of the group would mean you would need to make a set for EVERY member of the group.

Or you could make a set of items designed to be used by multiple people.

I've made a few paired sets of daggers/scimitars that share an attunement slot and power up a bit when you have both. It's a good way to offset the deficiency of two-weapon fighting.

I also like making them act as compasses pointing to each other, so once you get one you have a way to seek out the other.

I like the idea of party set items. Collect all the rings and you can summon Captain Planet!