You can tell Hugh Hefner was seriously beloved because even his former employees are rallying to celebrate his life. 

Our Playboy Mansion sources tell us ... staff members, new and old, started planning the reunion almost immediately after Hef died Wednesday. We're told 2 former butlers created a Facebook group that night to put the band back together -- employees, Playmates and Hef's children are all invited.

We're told the employees -- many of whom were featured in "The Girls Next Door" -- wanted to get together out of respect for Hef and the close-knit environment he created at the Mansion. Although many of them were laid off 2 years ago, we're told they still treat each other like family.

The staff event will be off-site from the Mansion, and is separate from the final tribute party to Hefner.