A person who has been invited and has registered with the Team Server is depicted with a colored circle with the person's initials in it. These initials are visible in the Team Server and in Enterprise Studio, for example in the timeline of a shared model package, but also when being invited to work on a model package or project.
If a person is not yet a (registered) user of the Team Server, the initials are not shown, instead you will see three dots. Below is an example of a registered user and an unregistered user in Enterprise Studio.
In the Team Server it looks as follows: