Members of a project are users who have been attached to it with a particular worker role. Those roles are :-
Owner - There can be only one owner, they have manager rights over the project, and this is the user to which the project will be allocated in the calendar. Changing the project manager in the general info page of a project will assign this, and normally promote the user doing it to sponsor.
Manager - Although there can only be one owner, it is possible to assign additional managers with similar modification rights on the project. Only managers and sponsors can change a project status (subject to workflow constraints), and only project managers can change key project data such as title, description or estimates. It may be desirabe to have additional managers for when the owner is not available.
Sponsor - Sponsors are created automatically when project ownership changes (the user doing this must have necessary permissions, and will be promoted to sponsor, however sponsors can be added manually). A project sponsor can change it's status (subject to workflow rules) and also has rights to authorize project change requests.
Member - Members of a project have limited update rights. They are not scheduled in the calendar for projects of which they are just a member, but can apply events and activities that accumulate time or costs to the project. Members are also included in meetings created within a project. It is quite likely that a project member may be assigned as the owner of sub-tasks or projects within a larger project.
Observer - Any user able to see a project can assign themselves as an observer, these projects will show under a tab in the user's home page. Observers have no rights over a project, other than to engage in the chat room (any user can do this). If a project is set to Eyes-Only, then any connected observers will be able to see it.