If you invited the additional people it is often implied they are your guests. If you said you were going to the steak house and the others said they like to come along, separate checks would be appropriate.
These days it's increasingly acceptable to be forthright and ask others to come along with a mention that you cannot 'treat' them this time given it's a spur of the moment event. Simple, to the point and polite. Then move on to another lighthearted subject.
Ask your gf to forgive you this time and reassure her that when you are forewarned and prepared to host guests you have no problem with doing so.