We assume you have the entire project and its structure ready.

Go to the selected project and to the structure tab.
Select the stage, task board / group to which you want to assign people. Click  to the right of the name of the given task board.

After you click it, a new window will pop up.

Click the field Assign user , select people who are to be assigned to the given task board. You can add all people to the project clicking  or you can assign / add people one by one by means of on the right side of the selected person.

After people are assigned to the task board, they receive an automatic notification: in the browser, by e-mail and via the mobile application.