You can add collaborators from the project's "access permissions" screen, simply @message them in a comment, or invite them from the Users page in settings.
In settings:
Inviting somebody from a project's access permissions window.
If you'd like to collaborate with another user who has a Reduct seat and is a member of your workspace, simply @mention them in a highlight's comment. If they aren't already in your workspace or your project, they'll be added automatically.
