It can sometimes be a difficult thing to know just what exactly to write in someone’s wedding guest book.
If you are the ‘plus-1′ of someone, you may not even know the bride and groom at all so pleasantries are what is called for in this instance. It is also acceptable to get your partner (the one who was actually invited) to write something and sign both of your names.
It may be that you want to spend some time thinking about a personal and heartfelt message, and are under pressure as the book is being passed around the tables, or their is a queue of people waiting to add their message behind you. In this case it is perfectly acceptable to take your time and ask that the book is passed onto you a bit later or you make a return visit to write something when the crowd has died down a bit. A word of warning though, don’t make it too much later when alcohol may have an influence over what might be written!
Remember that they will hopefully own this wedding guest book for the rest of their lives so avoid humour as this isn’t really the occasion to do your stint as a stand-up comic. Messages should only ever be positive, sincere, and personal to the bride and groom. If you know one of them better than the other, it is fine to dedicate your message to either the bride or the groom but make sure that you end the message wishing them both congratulations in their new life together.
If all else fails, a simple: “Beautiful wedding, I am very happy for you both. Congratulations and enjoy your honeymoon” will suffice.