Your gf may have drunkenly invited him, but you could’ve mentioned in that moment that you didn’t think it would be the best idea or would have to be aligned with the rest of the group first, rather than sending him the website and asking him to buy a ticket.

I am also not entirely sure why you don’t want him to come. If he is a good and social guy then what’s the problem? Him being glued to you is an assumption you are making. I have been invited by friends to join their group friends multiple times. I was never glued to them to the whole time. I was introduced, started making small talk, trying to get to know the people. It usually ends up in me talking more to the people I don’t know rather than the actual friend who invited me on those occasions.

Rest assured the guy is not going to expect you to babysit him during the event. And if you do find him being too glued to you that night then simply say “hé man, I’m gonna go talk to my other friends for a bit now”. Surely he would understand and will find his way talking to some other folks for a bit.

Idk, don’t underestimate someones social capabilities and just give the guy a chance rather than making all these assumptions. Just be chill and go with the flow that night.