There was already a thread about this
here, however I still say Steven Tyler, he's a good friend of both Mick and Stevie and a big fan of the band, including the pre-BuckinghamNicks formation; Aerosmith's live setlist have often included Rattlesnake Shake, Oh Well or Stop Messin Around and he's a pretty regular FM concert goer. He has an house in Maui as well and frequently hangs with Mick at his restaurant with members of the respective bands along too if they're present. In late March his partner threw a big surprise birthday party for him in his Maui house and invited both Mick and Stevie, so he may have already been there for a while.