Hi,
If helps to know, you can achieve this with Treekly app. We use it at work, it's brilliant!!
They allow users or businesses to create a community group (with your own branding if an organisation) which includes a fully automated Steps Leaderboard - with the option for a Team vs Team leaderboard too.
Users can connect any of the mainstream steps tracker services and wearables (eg Apple Health, Samsung, Garmin, Fitbit, Google Fit, and Strava) and it auto syncs the user's steps in the background. Therefore avoiding the thing you've referred to of people needing to 'start/end an activity'.
Best thing... you earn trees, planted on your behalf for maintaining over 5000 steps a day, which I found was perfect motivation for me.
They give 1 tree per month for free (completing 20 days). Basic level subscription option for 1 tree a week (Treekly!) or premium level is 1 tree every day you complete your steps.
My company regularly pay for us to do 30 Day Challenges which gives us 1 tree per day.
So I use Strava for my runs, and Treekly for my walks.
Hope that helps.