Justin Bieber to Guests -- This Party Will Cost You $3 Mil ... If You Blab!

Justin Bieber's Gatsby party Friday night was free of charge to guests, but if they utter a word about what went on inside they'll be socked for $3 million --- and get this ... it's a DISCOUNTED RATE!

Guests and workers signed a confidentiality agreement -- obtained by TMZ -- in which they agreed not to tweet, text, phone, Facebook, record, write or in any other manner spill the beans on what went on inside.

Apparently a lot went on ... Sheriff's deputies were called 3 times for noise complaints, and a neighbor tells us one deputy told him when the front door opened he caught a strong whiff of weed.

The document provides ... anyone who violates the confidentiality agreement will automatically be on the hook for $3 mil ... no trial, no arguing, just pay up.

People who worked the party were also required to sign the document.

For some reason, the Biebs reduced the penalty ... by a lot. TMZ obtained a similar confidentiality agreement he made people sign for a prior shindig, and the penalty clause was $5 million.