As of v9.0.
Multiple assigned users can be enabled for custom entity types and for some built-in entity type. Enable at: Administration > Entity Manager > {Entity Type} > Edit > check Multiple Assigned Users and save.
!!! important
Existing assigned users set in the *Assigned User* field won't be transferred to the new *Assigned Users* field after enabling multiple assigned users.
The maximum number of assigned users allowed per record can configured set at: Administration > Entity Manager > {Entity Type} > Fields > Assigned Users > Max Item Count. By default, it's set to 10.