"Hey, we're glad you're able to come to our wedding, but we're only able to accommodate invited guests. We are not allowing any extra people, as we feel this is the only way to be fair with everyone. I hope this doesn't affect your decision to attend."
This is what I told FH to say to one of his friends. Her boyfriend (who was legit invited, because duh) isn't able to make it, so she took it upon herself to just say she's bringing her sister instead. Um...say what now? Why...why do people still do this? It would've been different if she had said "Hey, my boyfriend is unable to make it, so would it be cool with you guys if I bring my sister instead?" But nope. Assume away please.