If you would like to add or invite a user to a scene, click on 'Members' on the main page of the scene, in the left side menu. 

The following fields will appear. 

List the username, name, or code of the users who you want to add or invite to the scene. 

Then select the users that you want to add or invite to the scene from the list of search results, and select the role of the new members. The users that cannot be clicked on and are grey are already members of the scene.