Re: Party crashers.
More than likely if someone asks me if they can bring someone, I will not say no. However there are events where I really want invited people only. Maybe because of the comfort level of other invitees.
The biggest thing is courtesy, someone who I have invited should extend the courtesy so I have no surprises.
I may take someone with me in the type of situation when someone is visiting me and I dont want to leave them alone, and if it is like one or two people for a smaller gathering, If Ihave a whole tribe visiting me than I may just extend regrets to some party I was invited to.
If someone does show up with people at an event, and it is an issue with the other attendees comfort then I may ask the guy who brought teh tagalong to do soemthing about it, otherwise not a big deal.