When creating a new deal at the bottom of the screen you have option to add partners. When you click on Partner field a dropdown with available partners will show and then you pick one you want to add.

If you want to add several partners to a deal you can do that by clicking on “+Add another partner” button. A new field will be created where you can complete the action. When finished with adding partners hit “Save” button and Partners will be invited to view the deal and all the other information you shared with them.
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When you want to add partner to an existing deal you can use green "Add Referrer" button among quick commands in top right:

Alternatively you can  go to "Edit Deal". You will be guided to the same page as when adding new deal.