Who is team member?
A team member is a placeholder.
In case you want to manage resources, you can do it without inviting people to your project.
You can always invite a real person instead of the placeholder. This person will automatically take over placeholder's tasks.
You can release people who used to be a part of your project by converting their accounts to the placeholder. This will remove person's access to your workspace but keep all their assignments together. Then all you have to do is invite a new person to join your workspace. You can remove a user from your Workspace for good, too.
You can add as many placeholders as you want since they're not considered active users.
Placeholders are free.