Project Creators can add collaborators with varying access to their projects in the Project settings tab of their project. Admins can invite a member to a public or private project. A public project is a project that is visible to all members of your contract. A private project is a project that is visible only to members of the project and admins on the contract.
You need to assign the user one of three permission levels (Full Project Access, Pipeline Only Access, or View Only Access). These permissions plus the users' contract-level assigned licenses and permissions dictates the actions the user can take in a project. Review the chart below to learn more about the access that comes with each permission level. Access to features that are also determined by the contract-level license and permissions are noted.