To act on Joker's suggestion, go to the kiosk at the bar on the second floor of the Silver Coast Casino and purchase Party Provisions for 100 credits (90 if both discount bonuses were chosen at the Intel Terminal). Afterwards, return to the office in the apartment and send out invitations using the private terminal. You can delay sending out the invitations and spend your time exploring the rest of the Silversun Strip instead, though throwing the party must be done, at the very latest, before Priority: Cerberus Headquarters.