So, this is strange, but the Fleetwood Mac Fan Club account on Instagram posted a pic of the band rehearsing for the MusiCares show. That photo has since been deleted.
Going from memory, I saw Stevie (with sunglasses) at the mic, John (back turned to the camera), Mick behind the drums, and Lindsey with guitar. Couldn't see Chris in the pic.
The Fan Club account was reposting James Robinson's photo. He appears to have taken it down is well, and it looks like he works for Suretone Entertainment, who appears to manage Fleetwood Mac to some extent? His account has several pics of him with Mick, Chris, Stevie. I apologize if somebody knows who this is or Suretone's relationship with the band, just did a little digging for my own education. This is his Instagram account:
https://www.instagram.com/kingjames508/ And here is Suretone's management roster:
http://suretone.com/management/
Anyway, just passing along what I saw!
EDIT: So sorry, disregard most of the above, here is the photo: