I've known my friend Justin literally for all of my life - we were born about 3 months apart. In honor of his recent marriage to his lovely bride Christienne, I dedicate this posting to them.
When you receive an invitation, take special notice. Is there a dress code suggested? To whom was the invitation addressed? If there is no dress code, such as "black tie", always opt for classic formal wear. For men, a well tailored suit, beautiful shirt, and tie will always be in style. Don't hesitate to wear colour, too. I usually take a hint from the colours of the invitation - they'll often coordinate with the theme of the event. For women, if the wedding is before evening (meaning before 5 PM), a knee length dress and cardigan or jacket paired with heels in a fun colour will never lead you astray. If the wedding is an evening affair, opt for something either more formal or more daring. Regardless, no one but the bride wears white.
Your invitation sets the guidelines for who may accompany you. "Mr. Joseph and Guest" would allow me to invite a lovely lady. It would not be in good taste for me to invite a lovely lady, my parents, my brother, my brother's fiance, and a friend who sort of knows the groom.
If, by some infrequent event, you bring additional guests than otherwise notated on the invitation, each additional guest is obligated to bring a gift for the lovely couple. And nothing cheap either. Weddings are expensive. You don't attend weddings for the free food. Or the free drinks. You go to celebrate the couple's union and be a witness to their lifelong bonds. So don't be a pig and scarf down plates full of food, or be that guy at the bar who gets wasted and ends up dancing and dry-humping the groom's mother.
And with that, I'll close with a toast to Justin and Christienne. May all that you wish for, be the least that you get.