This came up in a game the other night, it seems broken but I can't find anything that says it's not allowed.
A party member has "Find Greater steed" and casts it into a spell storing ring. Another party member then attunes, casts from the ring, and has their own Pegasus.
Repeat for the whole party and now they all have their own Pegasus to ride, until they dismiss it/it dies.
Obviously this assumes they have time to repeatedly re-attune and regain spell slots, but still seems a little OP. Have I missed something?