Salary is only a pay method and has no legal status of its own.
The following assumes that no contract or CBA exists which has differing requirements.
All employees, without exception, are required to attend whatever meetings the employer says are mandatory and to work whatever hours the employer requires them to work. All employees, without exception, can be fired for failing to do so.
If the employee is non-exempt, they must be paid for the time of the mandatory meeting, and if that meeting puts them into overtime status, the appropriate rate must apply.
If the employee is exempt, there are no circumstances whatsoever in which they are entited to be paid a single penny over and above their regular salary, regardless of how many mandatory meetings they attend or how many or how few hours they work.