We recommend reading 'Rules that govern permissions and roles' before reading this article.
To invite somebody to your project, you must first invite them to your workspace. When you invite a somebody to your workspace, you will select a workspace role that corresponds to specific access and rights. The actions they can perform in the project and workspace will depend on the role that they've been assigned.
To invite somebody to a workspace, navigate to 'Members' at a workspace level. Select 'Invite People' and enter the email address of the person you wish to invite along with the corresponding workspace role you wish to assign them.
Once the invitation has been sent, you can begin assigning the user to your desired projects and corresponding roles. At a project level you can then toggle access to certain environments also. You can begin this kind of user management before the user accepts the invitation to join the workspace and projects.
To read more about managing users at a project level, see here.