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From your project's sidebar, select Project settings > Access.
Select Add people.
Type a name, group, or email address.
Select the person's role in your project from the Role dropdown.
Project Lead, Project Team, Read Only - See permission scheme for the permissions/roles associated with each of those roles. In general, Project Leads should assign the Project Team role to the people working on the tasks in the project, Read Only to the people that they want to see the work being done, and Project Lead to other people on the team that they want to have the same permissions they have.
Select Add.