The people authorized to use the cabana will receive a wristband, identifying them as cabana guests. I don't know what Disney would do if they found you were purposely trying to get around the guest number limits, but why set yourself up for something that could be a negative experience. Since the cabanas on serenity Bay are not in a completely separate area like the ones on the family beach, perhaps your friends could have a group of chairs and umbrellas in front of your cabana. That way you could still enjoy time together.