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.