To Add a user to a project as a Collaborator, open the project, click the (+) plus sign at the top right and then select Add individual people.
Enter their email address, tick the checkbox then click Invite 🙌
The user will receive an email notification asking them to Join the project. Users must sign in with the exact email address that has been invited.
Until they confirm, you will see them on the Project as a Pending Collaborator. You can resend the invite if needed too.
Enable Project Sharing which will generate a url, send this link out so the user can add themselves to the project 👍
Removing a Collaborator from a Project:
Remove a user from a project by selecting the project, click on the plus (+) on the top right, select Manage Users and then select Edit. Click the (-) on each person that should be removed from that project.
(collaborators are indicated with a C):
Collaborators are added on a per project basis so they must be removed from each project they've been added to.