If you are an employer and have employees participating in today's called General Strike here are a few tips. The demands of the General Strike are not employment related: they are political and are not about to your workplace's specific terms and conditions of employment. As such this is not subject to the "no strike" clause of your collective bargaining agreement (if you have one), and is not a non-union "wildcat strike". As such it is not NLRB protected concerted activity in my opinion. You can take corrective action in accordance with your policies.
But be careful: this "is" protected First Amendment speech so terminating for absences related to this event could prove problematic. Still you probably don't want people walking off the job without consequences and/or disrupting your workplace.
Here is what you can do. Insist that the strikers leave your premises, not block your entrances and exits, and not disrupt your business. You can call in people to cover open shifts. You can follow your existing policies regarding unexcused absences including disciplinary actions and whether or not you allow paid time off for this absence. You should consider if someone called-in that they were going to participate, or if they were a no-call, no-show in deciding how to classify the absence.
Your best strategy is treat this event like any other absence, move on, and don't end up as a headline.