hooi -
I would suggest using SQL Server security rather then MS Access (my personal opinion) The reason being is that you can create roles (much like groups) and grant access to all objects in the SQL Server environment and you can also really limit access to certain objects. For example: whereas in MS Access you can grant users write permissions to a certain table in SQL Server you can grant permissions to a certain column in a certain table . So if a specific user group only needs access on one or two fields you don't have to grant them access to the whole thing! Also - you can use pass through queries to capture the users login and role in the front end to do all sorts of manipulation in forms/reports/etc... Furthermore I believe, (and someone please correct if untrue), that SQL Server security is more robust and secure then MS Access...
HTH,
Kev