Participants in a wedding fall into three categories:
Let's take these classes of participants in reverse order and see who belongs where, as well as some details about how to manage their participation in the event.
Many wedding services are typically provided by professionals who are complete strangers -- or at most, distant acquaintances -- to the bride, groom, and their families. These hired vendors do not receive a wedding invitation because they are not guests. They are staff, hired for the occasion to provide services that increase the enjoyment of the guests.
Therefore, the courtesy that applies to interactions with hired vendors follows the rules for business rather than for social occasions. The hired photographer, florist, and disc jockey know the wedding date and time from their contracts. For vendors who will work more than an hour or two, the contract should specify how breaks will be handled, and whether the happy couple should provide a "vendor meal" (caterers usually can manage a simple plate for vendors, at a lower cost than the reception meal) or the vendor will bring a sandwich. As valued employees, hired vendors are due the courtesy of reasonably comfortable work conditions!
There are two important potential exceptions to the rule that a paid vendor is treated as staff, rather than as a guest. First, if the paid vendor is also a social acquaintance who is almost a close enough friend to have ordinarily received an invitation, it is most tactful to go ahead and send the invitation. While etiquette experts recommend against having "working guests," this situation can be impossible to avoid in a small community where everyone knows everyone else. Long-time members of a church will find that they know the organist rather well, and may feel more comfortable including this person in the celebration after the ceremony.
Second, the officiant is almost always assumed, by courtesy, to be a social friend of the family. This assumption dates from the days when virtually all weddings took place in the bride's family's church, and the pastor had known the family for years. If the wedding ceremony is held at a church attended by the bride, the groom, or one of the two families, the officiant should be treated as a social friend even if he or she is a near-stranger. This means that the rule of "invite married couples together" is invoked, and the officiant's spouse is included on the invitation. Only if the officiant was a stranger hired for the occasion, and the ceremony takes place at a site separate from the reception, is it permissible to omit sending an invitation.
Certain people receive invitations even though they already know that they are expected to appear at the wedding. Both sets of parents and all of the attendants fall into this category, as ordinarily does the officiant. Providing an invitation is a courtesy that provides these participants with a keepsake of the occasion.
All of the ordinary rules for social invitations apply here. Adult attendants are invited with their spouses, fiances, or "serious" significant others. The line for "serious" is flexible, but the general rule is that, if Bob and Jean go everywhere as a couple, they ought to be treated as a couple, while if Alice is dating any number of people casually, it is up to the hosts to decide whether to encourage her to bring a date. If the attendants are expected to be available for numerous and time-consuming activities on the day before and the day of the wedding, some provision should be made for entertaining their attachments, especially if couples had to come from out of town (which means they have one car between them, with all that entails).
Parents of child attendants are ordinarily invited with their children, though this is not a hard-and-fast rule. When the child attendant is a child of the bride or groom, and the other parent would feel quite uncomfortable (or make everyone else feel uncomfortable), it is more usual to ask a grandparent or other relative to be in charge of seeing that the child is transported to and from the wedding, and supervising the child at the reception. But the child should receive an invitation in his or her own name.
Finally, there are ordinary guests who will not (we hope) show up without an invitation. Other than the rule that established couples must be invited together, even if you don't know or don't like one half of the couple, there are few firm rules on who must be invited.
In general, invitations should go to people whose presence the bride and groom will value. This can lead to difficulties if one aunt is much beloved, while another aunt is not so loved at all. It is rarely a good idea to make distinctions among relatives who are in touch with one another (so both aunts would ordinarily be invited), but it is not necessary to invite the groom's second cousins just because the bride is inviting the second cousins that she's genuinely close with.
It is desirable to keep "his" and "her" guest lists roughly equal, either in numbers or in closeness of relationship with guests. Often one family is much larger than the other, and this should be taken into account when dividing the lists. Parents should be consulted on the guest list, even if they are not involved in planning the wedding, as they can be a useful source of information on which relatives or long-time family friends are interested in the couple's doings. However, a wedding is rarely an appropriate time for the parents' business entertaining.
True "duty" invitations -- issued to people whom no one really likes, but who seem to have a right to an invitation -- should be few. A long list of duty invitations suggests that the size of the wedding is out of proportion with the pleasure anyone will gain from it. On the flip side, genuine friends who are assumed to be unable to attend should nonetheless receive an invitation as a gesture to show that the couple would like to have them there.