As I said in my previous reply, you need to make a call to getLoggedInAthleteActivities for each user. One of the parameters to the function call is the access token, so you would need to make the call with the token for each registered user. There is no way to get it all in a single call since all Strava API calls only return data for the user whose access token you are using, and getLoggedInAthleteActivities can return at most 200 results per call.
In pseudocode it looks something like this at a bare minimum
all_activities = [] //Empty array
foreach registered user:
token = get_auth_token(user) //Either from a cache if the token isn't expired, or using oauth + refresh token if it is
current_page = 0
do:
activities = getLoggedInAthleteActivities(page=current_page, per_page=200, access_token=token)
all_activities.append(activities)
current_page++
while (activities.length == 200) //Kind of a hack but it will work. It would be better to also compare the last activity timestamp against the 'after' argument
