Just looked at [NET-1075] Assign a user to several groups - Jira, i think, just looking at “User can be in several groups” is not was @marcinw meant … it’s about reference fields in general beeing able to store one-to-many (1:N) relations.
There is another post going in that direction: [ Thinking of a ‘n-Field’](Thinking of a 'n-Field'),
examples mentioned there:
n Phone-Numbers that are associated with an SIP account
n IP-Addresses that are associated to a Router
n VLANs that are associated to a Switch
n AV-Pairs in a Radius account
The final conclusion there “try to define the reverse” might help for some cases, but not always ;-).