To register the participants' attendance, select your group on the 'I’m sharing' section under 'My groups'.
On the tab 'Sessions', select the 'Register' option next to the session. Then, simply check the participants that were present during the session and click on 'Register attendance'.
You can always change this information later on: simply select the 'Registered' option next to the session you want to change, make the intended changes and click on 'Register attendance'.
