GatherPlace displays a Participant list to the Host while the meeting is running.
The participant list shows who is actively connected to the meeting.

GP4 Participant list.

Web host (Java) Participant list.

You can also run reports after the fact to see who was in the meetings, when they arrived, how long they attended as well as the email address they entered to join the meeting.
Please see this article for information on running reports on your meetings.