Go to Attendees and check the filters you can choose
Explanation of attendee email segments:
Before you can send an email, you need to decide to who you want to send it. You can do it by selecting a segment.
Seated = all people seated in a session (including hosts)
Not Seated = all attendees that did not choose a seat yet
Need welcome email = all attendees, excluding session hosts, that did not receive the welcome email yet.
Need first reminder = all attendees that received the welcome email, but did not take action yet (did not add themselves to a session/meeting)
Table host = are all the hosts of the sessions (for 1on1s, roundtables and workshops)
If you click on 'create segment' you can create your own segment with the filter settings. Read this article.