Just 'Public', that's all. Not a single additional permission or server role, because those are setup inside the NAV database. Just add the user and assign it to Public. Then you add the user to NAV security, and assign them to roles, which includes individual permissions. If you are on the enhanced security model, you will need to synchronize the user.
If you're on SQL 2005 or 2008, you also need to make sure that the role to which you assign the user also has rights over the schema. One of the more interesting nuances I've found about SQL security post 2000.
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