Go to Projects in the top navigation bar and click on the name of project you wish to update. Users in the project will be listed under the section labeled "Users in Project" and will have a "Remove from Project" link next to their names. Users not in the project will be listed under the section labeled "Account Users not Assigned to this Project" and will have an "Add to Project" link next to their names.
To add a user to a project, click the "Add to Project" link. To remove a user, click the "Remove from Project" link. Note that removing a user from a project does not delete the user, so you'll need to go to List Account Users if you wish to remove a user from your system.