Event attendees can be of two types:
–A mandatory attendee is a participant whose presence at the event is required.
–Optional attendee is a participant whose presence at the event is optional.
To add a required or optional attendee to an event, enter the attendee's email address manually in the Attendees or Optional attendees field or select a participant from the drop-down list that appears when you enter the first characters in the field. In the drop-down list, attendees who have already been added to the event are displayed in pale font, without the possibility of selecting them again.
You can also paste a list of addresses that was copied to the clipboard when viewing an email, another event, or a group of contacts.
If a group (distribution groups) is added as an attendee, there is a button in the box with the name of this attendee:

Click the button to view the list of group members. If needed, you can remove some of the addresses from the list of attendees in the event. To collapse the group, click
.
To change the attendee type (required/optional), drag the attendee name from one field to another.
Attendees with email addresses of external mailboxes can be invited to the meeting. After entering the external email address, press Enter or the Spacebar on the keyboard, and a bar with the entered address will appear in the field.
To remove an attendee from the guest list, click the button in the bar with their name or address.
To view the availability of invited attendees, go to the Scheduler on the tab.