Grandparents? Yes. Wedding party, immediate family of the couple, the spouses/fiances/live-ins/longtime SOs of any of the guests, and grandparents or the couple.

This definitely varies by community. In ours, for better or worse (no pun intended!), all the out of towners are always invited to everything for the whole weekend, including the rehearsal dinner (or whatever you call the dinner the night before, which is usually a casual party). But in others, only the bridal party and their significant others, and perhaps immediate family of the bride and groom, attend the the rehearsal dinner. You will find every variation in between, too. And people DO care: in our community, it's so universal to include everyone that people would be very offended if the hosts of the party the night before the wedding didn't include all out of town guests, all relatives in both families (because they want to be with the out of town relatives), and all friends who are helping in some way (hosting a shower or party, hospitality bags, baking, etc.). The guests would feel unwelcome, the bride's family (assuming the groom's family is giving the party) would be embarrassed (and maybe even plan some sort of dinner of their own to feed their out of town guests), and people generally would at best wonder if there were some sort of feud or other problem and at worst think poorly of the hosts. But in other communities, no one would think anything was amiss at all -- in fact, they would find it overkill to have any large party other than the wedding reception itself. (Miss Manners herself thinks that all the auxilliary events have gotten way out of hand.)

So it won't help to try to find a "rule" about "who gets invited." The hosts of the party -- often the groom's parents, as sheilajoyce points out, but not always -- decide whom to invite, just like any other party. But if they are in a community (geographic, ethnic, or otherwise defined) that always invites everyone, and they don't, no "rule" they read here or anywhere else is going to keep people they exclude from feeling unwelcome and thinking they are rude or cheap or both.

And irrespective of your community's custom, circumstances play a role. For example, say that in your community it's customary to keep the rehearsal dinner just to the bridal party and their spouses, and perhaps a few close relatives. If there are just a couple of people in from out of town, and they don't have other friends or family to entertain them that night, why not include them, too, instead of leaving them to sit in a hotel or find their own entertainment? Use your reason and your heart and you will know whom to invite.

As to your specific question about inviting grandparents, I would vote in favor of inviting them, especially if the others attending include the rest of the family, because they are in the bridal party. How are they going to feel if the rest of the family has a party without them? Besides, grandparents often seem to be sort of part of the wedding party even if they don't walk down an aisle -- lots of people buy a corsage for Grandma, for example. For many communities, weddings are very much a family event, not so focused on the bridal couple and their friends. Again, your individual family's circumstances will dictate the right answer for YOU.