We are looking for a way to add a picklist to an object that will reference an inactive user, or a free license user, like Chatter Free. We have a group of users who really don't need access to Salesforce, but we need a way to associate them with records without paying for unneeded licenses. We were hoping Chatter Free users could be referenced like Salesforce users in picklists, but it appears that's not possible.
Strangely, for some reason, you can assign inactive users to Territories in Salesforce, but you cannot reference an inactive Salesforce user or Chatter Free user in User picklists.
We prefer the user to be stored in the User object, but we are willing to create a separate object just for this group of users if we have to. Before we go through route, however, I thought I'd see if anyone else has found a way to do this.