A meeting called solely to issue discipline does not give rise to a Weingarten right. Note that a meeting, disciplinary at the outset, may become investigatory if management asks fact questions of the employee. The right to a representative is not automatic. The employee must ask to have another person present. Management does not have an obligation to inform employees of their Weingarten rights.
Management has no obligation to negotiate with a representative.
The representative may ask questions and otherwise reasonably participate in the interview but cannot interfere with management's investigation. The representative has the right, upon request, to be informed of the subject matter and to be given an opportunity to consult with the employee prior to the interview. The employer has no obligation to grant the employee's request. If the employee makes such a request, management may elect to forego the interview and may so advise the employee. For purposes of the NLRA, management may reach conclusions in the investigation without having interviewed the employee, but that could be problematic under other legal standards.