How should HR handle this situation of employees discussing pay?

They are openly complaining about their pay, and the clerical freelancer joins in that she has accepted below her normal hourly rate to work at this company.

Here’s the situation. 4 employees of a small office are sitting around after work. They are:

1 - the accountant

2 - the office manager (who does not actually fire, hire, or tell others what to do, but makes sure that projects are being followed up on and teaches employees how to perform tasks as needed)

3 - a professional contractor who has a desk and sometimes takes sidework with the company

4 - a new part time clerical freelancer who was brought on to do one work process

The account and office manager are both burned out. They have just realized that they are getting paid 10–20k below industry wages for their job duties. They are openly complaining about their pay, and the clerical freelancer joins in that she has accepted below her normal hourly rate to work at this company. The professional contractor is shocked that the accountant and office manage are getting paid entry level admin assistant wages, and supportively listens.

The accountant brings up how much former employees made, noting that the men made significantly more.

Who, if anyone, in this situation should get in trouble? What should the consequences be?