Technically, I don't think there is anything to prevent an employer for forcing you to attend company parties as a condition of employment. Of course the vast majority of employers wouldn't do this, but in this case your only choice is to go to the parties or take a different job.
Now an argument could be made that the employer-employee relationship is different from the union-employee relationship, but is there a reason that any such difference implies that a union can't set conditions for employment?