Someone may have found this out already but just incase anyone is wondering about having the repeating group scroll to the bottom whenever a new message is created (no matter the user), I did this using custom states here:
When the page is loaded, the repeating group scrolls to the last entry of the repeating group (the bottom):
Then I created a custom state on the repeating group called ‘Number’ (which is the ‘Count’ of the repeating group’s messages at the time of page load):
Then in a different workflow I created a ‘Do when condition is true’ (Every time):
And set the Repeating Group’s Number state back to the new count of Messages:
And then I think it should scroll to the bottom every time 
To test you can open the preview in a chrome window and another in an incognito window, scroll to the top of the group on each, create a new message in one window, and the repeating groups in each window should scroll to the last entry at the same time.
Editor: Forumapp3 | Bubble Editor
Preview: https://forumapp3.bubbleapps.io/version-test/scrolltoentry_messaging?debug_mode=true