No date announced so far. I expect foreign royals. Surely French and Italian ones, who shared her childhood and first years of adulthood in Portugal. Probably some Greek Royals.
Maybe some from reigning royal houses, too. I can imagine Princess Anne, since both were Presidents of the International Equestrian Federation. Maybe also Dutch royals, as well as from Belgium and Lux, as Catholic monarchies. Not likely royals from Nordic Monarchies, but who knows.