As an attendee, once you RSVP to an event, you may have the opportunity to send invites to others or even add a “Plus One” to join you if the Group Leader allows this feature.
A "Plus One" would make you responsible for their payment as they would be your guest. However, if you send an invite to another person, that person would then have the opportunity to create an account with GroupTools and join you at the event.
You may have this opportunity after accepting the invitation to the event again if the Group Leader allows attendees to invite others.
To send an invitation to someone you would want to be on the 'Event Overview' page. In the upper right hand corner you will see a button that is listed as 'Invite Others'. When you click that button, it will open up a window to allow you to enter the persons First name, Last name, email address, and your relationship to that person.
To add someone as your guest or +1, you would follow the same steps however before adding your guest, you would click the empty box at the top of the window listed 'My Guest'.