You've got the pen in hand, ready to divulge your love for the newlyweds, however nothing comes to mind. This is that sticky situation where you start quickly reading over other people's messages for inspiration, wishing you had thought of something beforehand to write down. Well, for the people reading this, you are probably the clever ones preparing beforehand, in order to leave a message that will leave them blushing and teary-eyed.
Below I will chat through some of your best options.

Formal Message
Many people opt for something formal, so not to appear odd to anyone else reading through them. Something romantic and heart warming will normally tick the box, with some potential messages below: Thank you for letting me enjoy this wonderful day. I wish you the best as you embark on this wonderful union. From now on you will always have someone by your side, you will forever be loved no matter what, congratuations on your special day. May your love last forever. You both should always listen to your heart, as this is what brought you two together. Today is the beginning of a magical journey. May you keep falling in love with each other again and again. All I want is for your life to be filled with unbounded love and happiness. 
Going soppy isn't for everyone. Many people will opt for something a bit more reserved. There is absolutely nothing wrong with that at all. Below are some ideas if you want to write something casual: Congratulations on your special day. Hugs and kisses. Really happy to be celebrating this day with you both. Best wishes on the future. I'm so happy for you. (If you have a gift) Here is a little something to start your life together. 
My favourite, you can't beat a funny message. Although most people go catastrophically wrong by thinking of a joke in the moment and come out with a proper 'dad joke', something you'd expect to pull from a cracker. As a general rule of thumb, avoid anything sarcastic or any rude jokes. Not everything translates as well written down and you may find yourself accidentally insulting someone, so keep it clean. Here are some ideas: Thanks for inviting us to get horrendously drunk while you get married. Love is all you need. That's what my single dad told me. Love is all you need...Remember that crap and you'll do alright. (Insert guy's name), remember to put the toilet seat down. Thanks for the free booze. That moment when you realise it's too late to run. Try not to mess it up. Congratulations on sleeping with the same person for the rest of eternity. Marriage lets you annoy one special person for the rest of your life. Can't wait to throw rice at you! Congratulations, you no longer have to hold in your farts. Remember the golden rule: Whenever you're wrong, admit it. Whenever you're right, shut up.  Marriage is a relationship in which one is always right and the other is the husband. May your days be filled with love and cake. 
If a family member is getting married, then you are welcoming a new member to your family, therefore it can be really nice to let them know you are welcoming them in with open arms. Congratulations and welcome to the family. What a wonderful day for our family! I couldn't be happier to call you a (brother/sister/daughter etc) I always wanted a brother/sister and now I get one. Lots of love to the best brother/sister in the world. I'm so thankful to have a brother/sister like you. 
If you're from a religious family then you might want to add something that will resonate with them. On the other hand, remember not to push your religious beliefs on others in the message, it can be painful to read, so remember to be reserved if they don't follow the same path as you. Below are some gentle religious messages you can post. May God bless you both on this beautiful day. Sending prayers for unending love and passion. May your marriage be blessed. May God grant you both true happiness. 
It doesn't have to be a book
The traditional option is to have a giant book for everyone to sign, but remember you don't have to go down this path. Many people are opting for creative ideas nowadays, which I much prefer. Below are some options which are available to you. Pebbles - You can order white stones, all around the same size, which are easy to write on. They will be limited to signatures, but it can look really nice. A globe - this is a great option if you plan on travelling far away for your honeymoon. Polaroid - My big sister bought a polaroid camera for her wedding and I went around to all the tables taking pictures of everyone. As the photo came out, I glued it to a book and then got people to write the message below the pictures of their group. Duvet - some people have begun signing duvets. While this might look good on paper, I'm not such a big fan, as it will probably get dirty at some point and need a wash, but hey, everyone to their own. Washing line - OK, I know you're already thinking 'What'?? Well, hear me out. If people write their messages on a piece of paper and then clip it to a line, then everyone can walk through them and read all the messages. It actually looks amazing. Jigsaw - get each member to sign a piece of jigsaw. You just need to ensure you have enough pieces, as that could get awkward. Video - I know personally I'd much rather watch a compilation video afterwards of everyone sharing their wishes. This could also be used in your wedding video as some extra great content. If you opt for a video, make sure to setup a camera on a tripod, so it's consistent and flows smoothly when it comes to editing. You also need to be wary of lighting and sound quality (maybe buy a microphone for people to talk into).