What is the owner specifically accusing your guests of doing? (I.e. is he/she accusing them of violating some house rules?) You need to know what their specific allegations are against your guests.

Also, has this person complained to management or the board about your guests or have they simply annointed themselves the 'co-op' police of the building?

As a Board President, there is very little the Board can do unless the other shareholder is formally complaining that you and your guests are violating house/rules and/or the Proprietary Lease. The Board will also probably be reluctant to get involved if it turns out it is just a dispute between shareholders where one is bothered by the fact that you have a lot of company. This is because anything that the board dicusses is typically recorded in your minutes and in this market you don't want a prospective buyer reading that the neighbors can't get along and as a result choose to buy elsewhere.

I would confront the shareholder directly and find out exactly what about your company is bothering them and ask them to refrain from harassing your company and if there is a problem to address it directly with you or you will be forced to file an aggravated harassment complaint against them with your local precinct.