You can change the guest limit in the 'Menu - Event settings ...' dialog.
'Event guests limit' parameter defines the maximum total number of guests including +1s, no matter if the guest is added by the host or RSVP by himself. For example, if your event limit is 15 guests and you already have 14 of them (including +1s) then the last guest would not be allowed to have any +1s.
'Max. allowed people guest can bring including him/her-self' parameter allows to define the limit for +1s of each guest. Please note that these parameters can be setup in the event settings dialog (Edit -> Event Setting ... -> tab RSVP):
Best regards,
Sergey
