You can expect a union representative, the union steward and one or more affected employees to attend. The union gets to select which employees attend, and often employees selected may not be those that will be directly impacted. Management can object to the number of employees if it impacts coverage or other operational needs.
Management may have one or more representatives attend. Generally, Department Heads do not attend, as they might be required to review any final decision that is made. Management also has the option of asking Employee & Labor Relations to send a representative to attend.