To join a public Plan: Tap the yellow “Ask to join” button at the bottom of the details page, and your request will be sent to the admin for review. Once you’ve been accepted, you’ll get a notification, and you’ll be able to message in the group chat to get involved.
For private plans: You’ll need to be invited to join by the plan creator. You’ll see a prompt within your 1:1 chat with a friend if you're invited. If you hit ‘Join Plan’, you’ll enter the group chat as long as the maximum occupancy of 15 hasn't been reached. Other members in the plan will also be able to see that you’ve joined. If you’ve been invited but you don’t want to join the plan, no worries. You don’t need to do anything.