Bobby Brown -- Planning Birthday Vigil for Bobbi Kristina ... And HIS Family's Invited (PHOTO)

Bobby Brown looked understandably drained as he left the Atlanta hospital where Bobbi Kristina remains in a coma -- and we've learned he's been busy planning a vigil for her birthday.

Bobbi Kristina turns 22 today ... and sources tell us her father has invited several of his family members to join him at the hospital.

That's unusual because while Houston family members have been at the hospital every day -- Bobby and his wife were the only family members from his side allowed at Bobbi Kristina's bedside.

We're told exceptions are being made because Bobby wants everyone to come together for the birthday.