Newsletter collaborators allow more than one user to login and edit your newsletters. You can have different collaborators for different publications.
On the Dashboard, Open the publication you'd like to manage users for
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:
Click the menu icon next to their name/email address and choose Remove User from the menu