Under the Administration view, go to Users&Roles then to Users.

Select the user you want to change access right, then go to the Project groups section.

There you can add, delete or modify access for this user to all Project groups available.

Once you're done editing user, click on the "Update User" button a the bottom-right of the screen. 

Add users directly to a group

You can also add one or more users directly into the project group window.

Open the project group you want to edit, then use the smart selector to select users, then click on Add user(s).

They will appear on a list below where you will be able to select their right level on the group.

When you are done click on Update group.