In your database, use one table to list your projects, and another to list your users.
If a user can only be linked to one project at a time, then their login will determine what project they have access to. If they can be linked to several projects, then you can show this in your database structure:
One table lists the projects, another table lists the users, and finally a third table links user id's against project id's.
When a user logs in, then show them a list of the projects they have access to and let them choose. A fairly simple join can be used to link in the 3 tables together.