Allow user access to only one or a few projects

How can I grant a user access to only one or a few projects ?

First, open the user manager and find the user you want to change. Make sure the "Full project Access" is unchecked.
Open "Project Administration" -> "Users"
You will see a list of user available to add to this project. These are users which do not have "Full Project Access". Add the users which you wish to give access to this project.

If you want to add the user to other projects, select a different project and repeat item two above.