I guess this would tie in with the “friend” option but it would be good to be able to tag folk as part of a running club and have a leaderboard specifically for that group of people.
I can already think of challenges such as admin etc. But throwing it out there 
There’s a related idea here of allowing people to finish a city together. I’m unsure if the idea made it into #ideas but the gist of it is that streets completed by any member of a group all pool together towards the percentage complete - potentially with competitions between groups.