Hello Justin,
Yes, you have to join user objects between your two AD, with a person object in the Metaverse. MIM is working with reference and needs to know the object to be able to add/remove it from a group membership.
If you see the users as string in the Metaverse, it means that you are not using a reference type attribute to store them in the MV.
From your source AD, you should project users as person in the MV, and from your target AD, you should add a join rules to do the Join (based on the account name for example).