To collaborate on a timeline with your friends & family you first need to make sure you have a timeline that's being stored in the app read more here
Once you have a timeline the timeline you want to collaborate selected you will see an collaboration icon in the top right corner of your calendar view
If you tap that icon you'll be taken to the share screen where you can enable collaboration on your timeline.
If you press "Send Invitation" and it to the person you want to collaborate with they will receive an link to join the collaboration. Once they tap the link Leap Second will open and they can now join your timeline.