There are drop down fields for adding sellers and HOA contacts to the listings details page. There are also 4 misc. drop down fields to add any additional contacts that are involved with the listing for your reference. These contacts are only able to be seen by you. 

Please note: When you accept an offer, the offer/negotiation page populates a closing section in which you are able to add contacts involved with the closing including lender, title, escrow, inspector and any additional contacts you wish to add. These contacts will be able to be seen by both you and the other agent involved in the transaction. Any notes you have on your contact that you add to this area will not be seen by the other agent.