How do I become a co-owner of a journal?

The owner of a journal (i.e. the person who originally created the journal) can grant Full Access to anyone that follows their journal, essentially making them a co-owner. Followers with Full Access have all the same editing permissions as the journal owner (except for the ability to remove the owner or delete the journal).

Here are instructions for how a journal owner can grant someone Full Access

From the app:

• Go to the More tab, select the journal, and then choose "Family & Friends"
• From here, tap the name of the follower you want to update and toggle on "Grant full access"

You can also update this setting from the Family & Friends page of the website.