You can invite both members and non-members.* These invitations are also referred to as Meeting Requests.
To Invite Attendees:
1) Open a new or existing event window. For an existing event, click Edit
2) To add a contact from your address book into the Attendees field, select Contact List from the tool bar and click on the name of the contact(s)
3) To invite a contact that is not in your address book, simply type a valid email address(s) into the Attendees field
*Members will be receive your meeting request directly their own account. Non-members will be invited via email.