If you are a member of a community, chances are you will be invited for an activity at some point in time.
Each activity invitation appears on your timeline, is sent via email and is posted via a push-notification on your smartphone (if you have downloaded and activated the mobile ThreeAndMore app). Within that message, you can select the option to 'accept', 'maybe accept' or 'reject' the invitation.
On your 'my calendar' page, you'll see an agenda with all the activities that are relevant for you personally. This means it only contains those activities you are (maybe) attending or still have an open invitation for. Within the calendar, you can click any activity to get a short summary.