The list of Users down in the Project Requirements specifies whose observations will get collected into the project. So anyone who joined as a member, but isn’t listed as a user in the project requirements won’t have their observations in the project.
Membership is controlled by the person signing up as a member. It means they can receive project journal posts in their dashboard, and could potentially be promoted to manager. It also means that if they have observations in the project, the observation pages will show the project badge.
The users in the requirements are controlled by the person who created the project, and those determine which observations get collected into the project.