The Hollywood Reporter have the list of attendees to the HBO reunion, marking 20 years since the first film came out.
Confirmed to be attending, so far:
Christopher Columbus (director)
Daniel Radcliffe (Harry Potter)
Rupert Grint (Ron Weasley)
Emma Watson (Hermione Granger)
Helena Bonham Carter (Bellatrix Lestrange)
Robbie Coltrane (Rubeus Hagrid)
Ralph Fiennes (Lord Voldemort)
Tom Felton (Draco Malfoy)
Jason Isaacs (Lucius Malfoy)
Gary Oldman (Sirius Black)
Imelda Staunton (Dolores Umbridge)
James Phelps (Fred Weasley)
Oliver Phelps (George Weasley)
Mark Williams (Arthur Weasley)
Bonnie Wright (Ginny Weasley)
Alfred Enoch (Dean Thomas)
Matthew Lewis (Neville Longbottom)
Evanna Lynch (Luna Lovegood)
Ian Hart (Professor Quirrell)
Felton mysteriously collapsed whilst golfing in September 2021 and was taken to hospital. Coltrane has been ill for years, is wheelchair bound and seldom makes appearances. Perhaps their presence should be enough to rule them out for next year. Then again, James Michael Tyler appeared at the Friends reunion this year via Zoom, and he was dead 4 months later.
Notably, J.K. Rowling has been snubbed, but that's more likely due to silly political reasons rather than anything else. Whilst HBO have said that other cast members will appear as surprises along the way, it's worthwhile noting who the omissions are for now:
Michael Gambon (Professor Dumbledore #2)
Maggie Smith (Professor McGonagall)
Warwick Davis (Professor Flitwick / Griphook #2)
Kenneth Branagh (Professor Lockhart)
Brendan Gleeson (Mad-Eye Moody)
Fiona Shaw (Aunt Petunia)
Timothy Spall (Peter Pettigrew)
David Thewlis (Remus Lupin)
Emma Thompson (Professor Trelawney)
Julie Walters (Molly Weasley)
Miriam Margolyes (Professor Sprout)
David Bradley (Filch)
Toby Jones (Dobby)
Leslie Phillips (The Sorting Hat)
Pam Ferris (Aunt Marge)
David Tennant (Barty Crouch Jr.)
More may ofc announce they're attending later, but worth keeping a tab on these...
Notable dead actors from the franchise so far...
Richard Harris (Professor Dumbledore #1)
Alan Rickman (Professor Snape)
Richard Griffiths (Uncle Vernon)
Robert Hardy (Cornelius Fudge)
Verne Troyer (Griphook #1)
Roger Lloyd Pack (Barty Crouch Sr.)
Helen McCrory (Narcissa Malfoy)
John Hurt (Mr. Ollivander)
(+ 28 others who are less notable)