A family of Microsoft relational database management systems designed for ease of use.
Again, YOU CAN'T, if you have converted to .accdb. The .accdb conversion destroys all your security settings so users are irrelevant. From the Help file:
"Also, if you convert your .mdb file to the new format (an .accdb file), Office Access 2007 discards your user-level security settings".
If you're still using a .mdb, in the Database Tools ribbon, under the Administer tab (on the right), choose Users and Permissions. You can add new users, create new groups, and so on. For more details, see the Help topic for "Set or change Access 2003 user-level security in Access 2007".