Given that you have sufficient permissions, to add a new user to a project, go to the Access Control page which is accessible under the
PROJECT SETUP dropdown in the sidebar. This page lists all the users added to the current project.
To add a new user, select New User. Add the email of the user you want to add under Users input. Under User Groups, you can select any of the existing user groups to which the user should be added. Lastly, add the roles for the user to enforce RBAC. Select Apply.
This will send an email invite to the new user, using which they can add themselves to the project.