Add/Remove Project Members
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 the input field to search for the desired user. The autocompleter of this field will deliver all matching users. Select the desired one and choose whether the user should be added as member or manager to the project. Click then on the "Add" button.