April Fool's Day

On April 1, 2007, Desiree Cartier is hosting a party at her mansion for her actress friend Torrance Caldwell. Present for a pre-festivities toast areâBlaine Cartier, Desiree's brother, who controls their joint inheritance (to Desiree's dislike), U.S. Senate candidate Peter Welling, Peter's "Miss Carolina" fiancee Barbie Reynolds, and the quintet's less wealthy videographer friend Ryan. With the party in full swing, Desiree's social nemesis and Ryan's not-so-secret crush Milan Hastings, arrives. Also present is chihuahua-toting society reporter Charles.
As another in her long string of April Fool's pranks, Desiree suggests Blaine get Milan tipsy on champagne and seduce her in his bedroom upstairs. Desiree and several of the others hover by the cracked-open bedroom door, prepared to videotape the affair with the camera Desiree lifted from Ryan. However, Milan has a seizure and falls off the balcony to her death. The group goes to court, and Blaine loses control of the family's fortune, which shifts to Desiree, but they are considered innocent and Milan's death is considered a fatal prank.
One year later, Desiree, Blaine, Peter, Barbie, Torrance, and Ryan receive anonymous invitations to meet on Milan's grave at noon, April 1, 2008, with the cryptic P.S. "I have proof." Just after the entire sextet is back together, a messenger comes to the grave with a box containing a letter and a laptop computer. The letter says one of the six murdered Milan, and if that person does not confess, all of them will be dead by midnight. As a show of intent, the computer has footage of Charles drowning in his pool.
Everyone goes from the cemetery to the pool to confirm this mishap. Subsequently, an increasingly frantic Desiree sees one "suspect" after another "murdered" before her eyes (though the bodies keep disappearing). Barbie is electrocuted in a beauty pageant dressing room, Peter's campaign truck runs him down in a parking garage, Ryan's throat is slit in his humble apartment, and returning home, Desiree and Blaine discover even their long-time butler Wilford has been butchered in the kitchen.
After a brief separation from her brother, Desiree finds him tied to a chair. Worse yet, gun-wielding Torrance soon has Desiree tied inches away in another chair. After some back and forth, and Torrance fatally shooting Blaine in the chest, a chagrined Desiree finally admits it was she who spiked Milan's fatal drink, while allowing brother Blaine to become the "fall guy." At this point Blaine cannot contain a chuckle, and soon the whole crew of 2008 "victims" are surrounding a still-tied Desiree, telling her what a bitch she is, and how they have conspired to prove it to her. Special effects people from Torrance's Boogie Nights 2 set have equipped Blaine and the others with "squibs," and other cast members faked Desiree out by dressing as cops and "confirming" Ryan's murder as she and her brother were fleeing the scene.
To illustrate her end of the charade involving a revolver shooting blanks, Torrance pulls the trigger once more. Unfortunately for Desiree, this time the cartridge in the chamber is real, and the bullet blows off the top of her head. The next scene involves the same inquest-probate judge from a year earlier absolving Torrance of any guilt for Desiree's death, and confirming Blaine as sole heir of the family estate. The final scene shows Blaine driving off in what was Desiree's red Mercedes, a slowly building smirk on his face.

Question: Who receive anonymous invitations to meet ?
Answer: Desiree, Blaine, Peter, Barbie, Torrance, and Ryan