How do you say welcome to a greek wedding?

Greek is all about the formulaic expressions. If you’re the guest in a Greek wedding, you must say:

• Να ζήσετε “may you live [long]” to the bride and groom.
• Να σας ζήσουν “may they live [long] for you” to the bride and groom’s families.
• Πάντα άξιος “[may you] always [be] worthy” to the best man. (You also say that to godparents.)
• Πάντα άξια “[may you] always [be] worthy” to the matron of honour.
• Και στα δικά σου “also [looking forward] to yours!” to anyone present and unmarried. A phrase that has made not a few unmarried Greeks choose to stay away from weddings.

Now, if you’re the bride and groom… actually, if you’re the bride and groom, you don’t traditionally say all that much, and I’m not aware of a formulaic expression of welcome to a wedding. The generic καλώς ορίσατε “welcome” will do, if you have to say anything; but my recollection is that newlyweds mostly just beam a lot, and dance.