If you fancy a group activity, give your friends a quick shout via Racefully chat (in your Tribe or 1 on 1). Agree on the date, time and who will host the activity.
If you're the host, create the actual group activity right before your agreed start time. Then your friends will be able to see and join your group activity in their activity list.
*Not sure how to create a group activity? There's an article for that, too!