HAR on MLS, go on edit tab listing and add+ Where your information is all for MLS listing add Co agent name, I am sure can be auto populate after that, or add other agent MLS public ID and license number and cell number. Check with your Broker also and I am sure its your Office agent and no need new listing agreement, and other documents (referral fee) etc.
You can add a Co listing agent in Matrix on ADD & EDIT. By going to the showing information section and you will see a drop box that says “Find a Co List Agent” once you put the agent Id it will auto fill the agents information. Hope this helps!
You can add a co agent real easy but I only know one way and that’s to terminate it and copy mls number over. Then you will have to pick the listing agent from the drop down. Now if it was a mistake and just put on Har wrong they will fix it for you but they will make you send them the listing agreement. Good luck!