How to post Apple Watch activity of a group to slack?
Since health data is highly private, you’ll want to have an app run on each device so everyone opts in to the collection. 

This has been done very well - https://apps.apple.com/us/app/challenges-compete-get-fit/id1051342211

I would start there and then see if you even need slack integration. The shortcuts app does have the ability to pull data from the health database, but you’re going to need to connect a few things and then architect the trigger or have people periodically push their data. The app above has already solved all those “design and technical challenges” for you so you can all just design a challenge on workouts and fitness 



Here’s how you get started with shortcuts. You can chain them together and call any web API so that’s how you’d get things to slack if there isn’t a slack app that works with shortcut automation on iOS. 

There is an app for slack you could install and see if shortcuts still can post to slack after the app was renamed
from workflow to shortcuts. 


https://your-slack-instance.slack.com/apps/A03A1LEM4-workflow