The right contract is important in setting up good performance management and employee relations for the future.
the theory of psychological contracts in the workplace and in the wider world outside work.
If you find yourself with personal friends in teams you lead, I would recommend an early discussion with the friend about the ground rules. I believe you need to be completely honest about how you intend to play it. Reassure others there will be no favouritism and, my word, you will need to make sure you don’t show it.