Well it's your party so invite whoever you want. Even if it's people you know but not very well because it's a 50/50 as to if they will really actually come but you are the nice person that sent an invitation and it makes people feel good to know that they are included even if they are not as familiar as most. For a wedding it is customary to have +guest on your invitation. That way the work is up to them to decide if they want to bring someone or not so invite who you know and the +1's will fill in the blanks of who you were not sure about. For a company party keep the same code. Add a +1 or and spouse. Invitations must be given to significant others at company parties because if some are invted on a personal level and someone's wife or husband is forget then it can really put a damper on a relationship. Same rule goes for engagement parties, housewarming parties, religious parties, even generic parties