Okay... I figured it out.

I enabled both the "Enabled" and "Required" flags in Team Management

When Team Managment is enabled each user is created as his/her own Team. So User "John Doe" is the team John Doe... one member.

I can create a Team of many members as well. E.g. Sales Team and add John Doe, Sue Smith, Mary White and Jack Frost

Then on Leads, if I only want John Doe to have the lead, I assign it to the team  "John Doe"

If I want the Sales Team to have access, I assign it to the Sales Team and all members will be able to view/edit the lead.

I ignore the "Assign To" field on the lead.