In Pipedrive, a deal can only have one linked contact person at a time. However, if a deal has more than one contact person related to ongoing processes, you can add these additional people as a Participants to that deal.
Participants are not formally linked to the deal in the way that the main contact person is and won't appear in the List View or in reports, but serve as an easy reference for the contacts who may be important and related to the deal while in the deal or contact's detail view.
To view and add Participants to a deal, go to the Participants panel of a deal's detail page. There, you can view or add new contacts as participants to your deal by clicking on the '+' icon.
You can learn about how to add participants in bulk in this article.
If you click through to the Participant's detail page, the deal will appear under the Participant section of the related deals.