What is the difference between an attendee and a user?

An attendee is basically a user who is added to a meeting. Let's see a quick example:

• User A invites users B and C to the meeting 1. This meeting has 3 attendees.
• User B invites user A to the meeting 2. This meeting has 2 attendees.
• User C invites users A and B to the meeting 3. This meeting has 3 attendees.

Globally, in this Team there are just 3 users (User A, B, C), although they are attendees to different meetings.