It is my understanding that the groom's parents pay for the rehearsal dinner, my question is this, "exactly who does this include?", bridesmaids and spouses, out of town guests as well, the parents of the ringbearer, etc. or are they responsible for picking up the tab for just the wedding party members? this can be quite expensive if everyone is included.
Typically, the grooms parent's do host ther rehearsal dinner. As the hosts they would pay for anyone attending. You should invite everyone included in the wedding ceremony (ushers, bridesmaids, bestman, maid of honor, flowergirl, ring bearer, officiate, etc.) Spouses or dates of all adult attendants should be invited. Parents (divorced parents new spouses or partners), grandparents and siblings of the bride and groom should be invited. If children are involved in the wedding, the children and their parents should be invited. If there are out of town guests staying over it is considerate to invite these people as well, however not required.
This could get expensive deopending on the restaurant they choose, IF they even choose to hold the diner at a restaurant. They could have the dinner at their home (if they have the room) and have it catered. I have even heard about some who have a potluck supper, not my first choice but you do what you have to do sometimes, right? Top Wedding Questions Forum Moderator - "Write your sorrows in the sand, your blessings in stone".