Newsletter collaborators allow more than one user to login and edit your newsletters. You can have different collaborators for different publications.
On the Dashboard, click Settings > Users from the left menu
Here you can invite other users to collaborate on your newsletters. They will have full access to modify any of your newsletters.
To add a user:
Enter their email address and press the Add User button
They will be sent an invitation link to their email where they can finish setting up their account (and setup their password)
To remove a user:
Place your mouse cursor over the name of the user you wish to remove
Click the trash icon next to their name