Most likely, you, your fiance, and both your families will be contributing names to the guest list, and each party will have his or her own ideas about who is necessary and who can be eliminated.
Start by setting a goal for your list size. This can be based on your budget, on space limitations of the site you want to use, and/or on any other factor that's important to you.
Have everyone involved in the inviting submit a list of the people they'd like to attend. They should rank the names in order of importance, starting at the top.
Compare the lists and cross off any duplications.
Add up the names that are left and compare that number with your goal. If your count is over the limit, determine how many names you need to eliminate.
Then, have each person cut an assigned number of names, starting from the bottom of his or her personal list.
It can be tempting to limit your list by not inviting your single friends with guests — and that's just fine, if they truly are single. But if they're married, engaged, living with someone, or in a long-term relationship, you must invite their significant other (yes, even if you've never met him or her). Not only that, but you should include the significant other's name on the invitation — don't just write 'and Guest' if there's a specific person in your friend's life.
Including children in your guest list is an entirely personal choice. Some couples couldn't imagine getting married without their adored nieces and nephews in attendance; others don't want the sticky little rugrats anywhere near their elegant affair. Children's meals are often less expensive than adults', but kids do take up as much space — or more, if you set aside a play area for them. Not inviting children can be an efficient way to eliminate an entire block of guests, and though some parents may be offended, they shouldn't expect you to invite their kids. (Just be sure to apply the decision uniformly, and don't make exceptions.)
You probably spend more waking hours with your office-mates than with your future mate, but does that mean you need to invite them all? Don't worry — unless you're close enough friends that you already socialize outside of the office, they probably don't expect it (and may even be glad not to have to give up a Saturday night).
If you do decide to invite coworkers, remember that you'll need to invite their spouses or significant others too, as described above. And if you don't invite them, stop bending their ears about wedding plans all day!
One way to make sure you invite as many people as possible but don't exceed your desired number is to have a back-up list of people you invite only after you receive regrets from your primary list. This means you need to send your A-list invitations out eight to 10 weeks in advance. Have the B-list mailing ready so as soon as you start receiving negative A-list replies you can drop them in the mail.
The only real downside to this approach is the risk of hurt feelings if a guest realizes he or she wasn't a first choice. To avoid this, make sure nothing on the invitation or envelope indicates A or B, and don't send out any invitations too close to the wedding date.