As parents of the groom, several of our guests were unable to make the trip to the brides hometown for the wedding. We plan to have a house party to give them an opportunity to meet the bride and groom 1 month after the wedding. We don't want them to be obligated to bring a gift, but don't know if we should mention that on the invitation or leave it up to the invitee to bring a gift or not. Several of the invitees have sent gifts even though they could not attend. Others have not. Some were not invited to the wedding, but we still want them to meet our son and his new wife. What is the etiquette for this?
The etiquette is that you host a party, letting people know the party is in honor of the newlyweds. Naturally, you want your family and friends to meet your son's new wife.
Don't say anything, positive or negative, about gifts. Gifts are always voluntary, anyway. If someone wants to give a gift, they will. If not, they won't. The only thing you need to do is have a place ready to put the gifts as people arrive. Clear off a table and put a nice tablecloth on it, as well as (perhaps) a bouquet of flowers.
It always bothers me when people try to proactively direct gift-giving. People can figure out for themselves whether or not they want to bring a gift. If anyone asks about a gift registry, just direct them to the already-existing registry from the wedding.