Hi Laura & welcome to the CityStrides forum!
In his original post about adding the “Challenges” feature to CityStrides, James did say this is a supporter-only feature (see first bullet point). I did a quick check of the biggest active challenge, 2022 streets in 2022, and every one of the first ~20 users in the list are supporters, suggesting this hasn’t changed.
As you probably know, your friend could become a supporter for even just one month. Besides joining you in your challenge, they also get access to a ton of great features, and so much else! 