Is it a SQL authenticated user or a Windows Authenticated user?
If Windows, create the login on Principal and Mirror. Map the user on Principal to the database
If SQL, create the login on Principal, use something like DBATools or sp_help_revlogin to script the user to then be applied to the Mirror. Map the user on Principal to the database.
Once all synced between principal and mirror, drop the snapshots, re-create the snapshot.