From your Prioritizer, or anywhere else in ResultMaps, here's how you can add a teammate to an existing item:
Click the title of the item you'd like to team up with someone on to open the item's detail window.
In the menu of icons, select the People icon.
Enter a team member's name into the text-box to team up.
Select their name from the suggestions drop-down
A box with their name will appear in the text-box, which means you've added them to your item and they've been sent your team-up request.
From anywhere that lists action items, you can click on the item title to open the detail window and click the People icon to add a teammate.