You can add additional users to your workspace from the Users page in Settings, from a project's "access permissions" screen, or by simply @message them in a comment.

Access settings by clicking on your profile picture in the top-right corner of the screen, and choose Users on the left. Then click on the New User button to add one or multiple users to your workspace.

Inviting somebody from a project's access permissions window. If you invite the new users as Editors, they will have access to the entire workspace; if you add them as a Guest Editor, they will only have access to the project(s) you explicitly invite them to.

You can do this from the project listing page:

Or when you are already in a project, using the button marked "Share" in the top-right corner of your screen:

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.

For more on the different roles in Reduct, and their access permissions and abilities, see this article.