Restoring Database And Losing Users

I backed up and restored a database successfully except for the users. The only user that shows is dbo, but I can't add a user with the proper user name from my old server; I get the error that the user or role already exists in the current database.

When I go to role, name of public, there is the user I need to add. I can't remove it as it's a table owner, but I need to add it to the list of users, so I need to remove it then re-add it.

I tried using EXEC sp_changeobjectowner to no avail.

Thanks for any help/guidance.