I'm still a newbie, level 8 I think, and still don't know how to do everything on there..

So when I first started just riding today, as a warmup for a race I was scheduled to do, I saw a friend online riding via the companion app, and there was a button to join them in their ride, pressed it and it put me on course right behind them.

Then I left after a while, did my race, and went back to just riding. When I looked in the companion app, I could see my friend still riding, but there was no button to join them, rather just a button to request a meetup. Why did the join them option seem to disappear? They were still riding...