What is a common blessing given to the bride and groom when attending a Jewish wedding?
What is a common blessing given to the family and friends of the bride and groom when attending a Jewish wedding?
To the bride and groom, commonly I have heard (and given) the blessing: "May you be zocheh to build a bayis ne'eman b'Yisrael!"
To the attendees (from the bride and groom), if they aren't married and are of marriageable age, it is common to give them a blessing to find their mate. If they are married, I haven't really heard any "standard" blessing.
To the family and friends from other wedding attendees, other than "mazal tov!" (which is a form of a blessing, to which many people respond "amen"), I haven't really heard any blessings given.
As far as I know, there are no more beautiful blessings to offer the couple than those included in the Sheva Brachot recited under the chuppa as well as at the end of the wedding. If that's insufficient, for many couples, it's said multiple times throughout the following week, if they have Sheva Brachot meals. Look here for the text in Hebrew, translation and transliteration. Look, esp. at blessing #6 which calls the couple "beloved friends" and wishes them that they should be as happy which each other as Adam and Eve were (not directly, but it's metaphorically implied.)
As for what people wish friends, the most common saying I have heard the host tell guests and friends is "May we share numerous future simchot together." I think it's a great blessing of optimism for the future as well as the hope that each guest has cheerful future events and that the friendship will last long enough so that the host can return the "favor" by being present at the guests' events.