The only thing I objected to in this whole thread was no pardon's decree that no single person should be invited to decide to bring a guest.... "So and so, and guest" is a very polite way to invite if you aren't sure. When they RSVP, they'll hopefully let you know if they are bringing a guest. But if cousin shows up with someone you didn't expect, and someone tried to throw them out, or tried to enforce some silly arrangement about where to sit, I'd be very ashamed. That isn't what's important.
It's not something I would do should it happen. However, I read there are ways to word an invitation so the likelihood of that happening is reduced. If an invitation doesn't say guest, a person should not bring one but if it says a guest may be brought along, I guess it's okay.
The "and guest" may be more and more usual today, but that doesn't make right.
More and more people here write "anyways", which isn't even a word, when they mean ANYWAY. Because it is now used more and more, does that then make it correct?
And if cousin so and so shows up with some stranger, THAT IS RUDE and while a place and food would somehow be found,I guess.But it would NOT be me, who would be ashamed at having him/her thrown out. The cousin should have alerted whomever was throwing the wedding...even IF it was a very informal wedding.