Maybe its different because we're a small company, or maybe I'm deluding myself, but I've always run 'work parties' here as non-mandatory and people seem to see it that way without trouble. Typically we'll buy pizza, or have a bbq for lunch, or handle birthdays by one cake a month for everybody who has a birthday that month, and the employees who want to partake do, some never do, and some do occasionally. Nobody gets ostracized based on whether they attend.

But I also 1) never provide alcohol, 2) never require any social 'ice breaker' type games, and 2) never have company events during non-working hours.