Another great article from www.elegala.com on Invitation Etiquette
Inviting partners and guests
If an invited guest is married, engaged or living with a significant other, that partner must be included in the invitation. A single invitation addressed to both individuals should be sent to spouses or couples who live together, while separate invitations should be sent to each member of an engaged or long term couple who don't live together. Inviting single guests with a date is a thoughtful gesture, but one that is not required. If you are inviting a single guest with a date, try to find out the name of your friend's intended date and include that person's name on the invitation. Otherwise, inner envelopes may include "And Guest," indicating that he or she may bring any chosen escort or friend.
To invite or not invite the little ones - this is a situation that can quickly get ugly. Make your decision and stick with it - then inform your guests through carefully addressed invitations:
Children over 18 who are invited to the wedding should receive their own invitations - regardless of whether or not they live with their parents. If you don't send them an invitation - it's clear that they're not invited.
Children under 18 who are invited to the wedding should have their name included on the invitation. If you're inviting Joe and Mary Smith without their two little ones, their invitation should read "Joe and Mary Smith."
If you're still worried that some guests may add write-ins on their reply card - print the names of those invited on the reply card as well.
Guests Who Ask to Bring a Guest
Your guests should know better! It is never appropriate for a guest to ask to bring a date, and you have every right to politely say no. However, if you discover that a guest is engaged or living with a significant other, you should extend a written or verbal invitation.
Invitations to out-of-town guests
Many brides ponder whether or not it's appropriate to invite long distance guests for whom it may be impossible to attend. Use your best judgment. Is this person truly a close friend who would want to attend your celebration? If so, failing to extend an invitation may be insulting. Remember, these days friends and family are often spread all over the country, and people are accustomed to traveling. On the other hand, if you haven't spoken in years, an invitation may look like no more than a request for a gift. In those cases, send a wedding announcement instead, which carries no gift-giving obligation.