In response to your inquiries, WeddingChannel.com's etiquette expert Peggy Post has chosen to address this interesting and complex question.
Q: My fiancé would like to invite some co-workers and acquaintances to his bachelor party who are not invited to the wedding. He claims they know they'll be missing the main event, but would like to celebrate with him anyways. Is it ok to invite people to wedding-related celebrations like engagement parties, showers, and bachelorette/bachelor parties who are not invited to the wedding? 

A: As a general guideline, everyone invited to pre-wedding festivities should be included in the big event. But there are some exceptions. Invitees to events with gift obligation -- namely bridal showers -- should be invited to the wedding. It's a major snub to invite someone to a shower and not the wedding. The exception includes a shower thrown by coworkers who know ahead of time that they are not invited to the wedding.
A shower invitation bears with it an obligation to purchase a gift -- whether the invitee attends the shower or not. When couples marry far away from where they live and work, they are often feted by coworkers and sometimes friends who have no expectation of being invited to the wedding. Consideration for the feelings of others and delicate communication are the keys to navigating these situations where people celebrating your happiness ahead of time may not be invited to the wedding.
The same holds true for engagement parties. The majority of engagement parties are limited to relatives and good friends. The people invited to the engagement party are often your top-priority wedding guests.
The rules around bachelor and bachelorette parties are a little looser. These celebrations more often take the form of a night on the town, a luncheon, or an afternoon at the spa -- and carry no obligation to purchase a gift. But, here as well, it's important to let any guests know ahead of time that while you hope they're able to celebrate with you, you can't invite them to the wedding. Be prepared to discuss it up front: "Jason, you know we've had to keep the wedding guest list short due to space. I'm disappointed that I can't invite you to the wedding. But I'm getting together with some of the guys next Tuesday for a bachelor night out and am really hoping you can join us."

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