Obviously it’s quite redundant, because “you can just click on someone’s name” (or click on ‘Create’ and then on the smaller button in the bottom right…), but perhaps it’s better to give people a big fat button to click that’s easy to see and understand, instead of having to ask about an otherwise ‘hidden feature’ all the time. Whether it’s just ‘challenge’ or ‘challenge (a/nother) user’ or whatever, I’ll leave up to you. 
It would also add a buddy to the lonely ‘Create’ button and make the whole thing symmetric. \o/