AFAIK, currently there's no way for doing this. I've opened several threads to try find ways of making this work but had no response:
Here I requested adding an endpoint to the API so a logged in user could fetch the activities of the athletes he/she is following:
https://communityhub.strava.com/t5/developer-discussions/following-athletes-activities/td-p/9374
And here I propose adding the Athlete ID to the club activities. This way, as a member of a club you would be able to list the activities for your club and therefore correlate them to their athletes:
https://communityhub.strava.com/t5/developer-discussions/clubathlete-does-not-include-athlete-id/td-p/9378
The only solution we've find for this so far is having all athletes granting access to your app. Then you can either listen to the webhooks or poll periodically for activities. But IMHO this is the worst user experience for a lot of scenarios.