Amazon.com has a book which has a 4 star rating:
The Perfect Stranger's Guide to Funerals and Grieving Practices: A Guide to Etiquette in Other People's Religious Ceremonies (Paperback)
They typically don't serve liquor at the wedding and even though they refuse to salute flags and accept things like blood transfusions, that shouldn't be a problem at a wedding.
Jehovah's Witnesses celebrate no civil holidays and few Christian ones, but that also would not affect the wedding unless you asked them if they had a nice Christmas or Thanksgiving.
Traditionally, a modest and reasonable wedding is encouraged, while lavish weddings are frowned upon as status symbols. Other traditions are similar to those found in other Protestant faiths, such as bridesmaids, groomsmen, the exchange of vows and a ring ceremony. The reception is held either at a private home or hall, and features traditional wedding cake, music and dancing, according to the couple's wishes.
A JW gave this response to your question:
"Dress in a moderate way,not too flashy, and not too casual. A nice dress,not a mini skirt,would be appropriate.Not alot of make-up or purple colored hair(any attention drawing adornments such as tatoos or alot of piercings)these things you would want to avoid,respectively.
Customs of the world are not part of a JW wedding such as throwing of rice, there is usually a exchange of a ring, but it is not a Biblical or Scriptual requirement. No cell phones please! Other than the two getting married as JW's,it is basiclly the same as someone elses wedding in a church. If there is a reception afterwards the same applies,not traditional as others, with smoking, drinking, dirty dancing, loud music,etc. Just go with the flow and you should be fine and will have a vey good time. If you still have questions on this, don't hesitate to ask a women or two who are attending,or ask the bride to be to set your mind at ease.I myself would dress in a suit with a tie,the same way I dress when attending our Kingdom Hall during the week.Have fun and enjoy yourselve,we are not party poopers,but there is a limit to our activities we have."
Hope this info helps you. It doesn't sound too complicated. You'll be fine!