Why wouldn't you want to go? If friends or relatives are getting married, and they happen to be Wiccan, of course you would want to be there!
Don't worry, chocolatepudding, about making mistakes. Wiccan ceremonies are usually very informal and there is rarely any expectation of "doing the right thing.". As long as you are loving and respectful, you'll be fine. A handfasting, or marriage ceremony, will usually involve an exchange of vows, a toast to the God and the Goddess and to the happy couple, and nearly always lots of flowers, food and wine! There may be a symbolic action, such as jumping over a broomstick or a small fire, though not always.
Have a lovely time. I'm sure you'll enjoy it.