Go to members section of the "Show Project" screen. To add a user as member or manager of the project, click on the "Add User" button.
In the "Add User" screen use an input field (green outlined box) to search for the desired user. The autocompleter of this field will deliver all matching users. When you found the correct user, think about, whether the user should be added as a member or a manager to the project and then click on the " Member" or "Manager" button.
Remove a Project Member.