You can add others to teams which gives them editing access to specific Yapps. Note that you will need to be subscribed to a Team plan in order to share access to an activated app.
To invite others to a team you've already created, open that Team dashboard and click 'Manage Team'.
From there, you'll see on the right side a list of Team Members with an 'Invite Members' link. Click that, add emails (separated by commas), and the invitations will be sent out.
When members accept, the name associated with their Yapp account will show up. If they haven't yet accepted, it will show their email in gray with 'pending' after.
Once members have joined your team, they'll be able to access the apps that you store within that Team account even if they do not have their own paid account. If you've already started an app in your account that you'd like to share with a team, you can easily move it from one dashboard to another.