To collaborate on a journal with your friends & family, the user sharing the journal will need a Pro account.
While in Calendar view, select the journal you want to share from the dropdown at the top of the screen and click on the collaboration icon that appears in the top right corner.
If you tap that icon you'll be taken to the screen below where you can send a link to collaborate on your journal.
Pressing "Send Invitation" will create a link that you can send via text message or email. Once your collaborator taps the link, Leap Second will open and they can now join your journal.