loggin into linux account

Hi, The scenario is, I made a login interface wherein i accept usernames and passwords from users and after comparing them to a database I log them in. The server runs Linux Fedora Core 2. Now I want to know whether there can be a scenario wherein I can make the logged in user have access to files of which he/she is the owner of or the file belongs to a group whose membership he/she has. Is this possible ? because as far as my knowledge goes, its only possible to give access to users to specific files if the user logs in to a particular shell. Please provide some help on this if you can. Thanks in advance ! Thanks Vaibhav