The user could not be added to one or more databases. Dynamics GP

This error usually goes away by deleting the user login from the security area for the specified database in SQL. Then re-add the user to the User Security in GP.

If that doesnt resolve the issue, the below will help:
In SQL Server Security=>Logins=>User Mapping, to uncheck the Map checkbox on the restored company.  After doing that the User Access function in GP will allow you to add the users to the company.