Thanks Jason - that makes sense I guess. I have another site which is more like that, where all users are employees of the company, with administrators with rights to all admin pages, and editors who are able to edit records.
This one is a little different in that the administrators are the site owners, and the other users are third parties who can log in and add their own tour itineraries to the site.
But yes - now that you mention it groups will work - the fields for the administrators are just a subset of the fields for the third party users, so one table should easily cover both groups.
As its still at an early stage it should be easy enough to combine the tables. So I'll do that in the first instance and come back to you if I have any questions about setting up the user level authentication.