In the 1975 United States Supreme Court decision, NLRB v. Weingarten, Inc., the courted ruled that public employees do not have to be alone when questioned by employers if they feel it may result in disciplinary action.  

As a member you have the right to be represented by a union representative in any meeting. If you do not know why a meeting is being called, ask in advance of the meeting. If a meeting is scheduled at a time when a building representative cannot meet then reschedule the meeting. If you get into a meeting and you realize it is an investigatory meeting, it is ok and advisable to ask that the meeting be rescheduled when a build representative is available.