It sounds like you will be required to keep your mobile device with the controller during follow-me. This means you can't just throw a phone in a mountain biker/skier's pocket and tell them to go (while you keep the controller to take back control if something goes wrong).
From 3DRs website, regarding "Follow" (emphasis added):
It’s this easy: Launch Solo and choose Follow. Now Solo is locked onto your mobile device—which must remain with your controller—and you’re ready to film all the action. And since flight and camera control are fully automated, you’re completely hands-free to focus on the activities you love as Solo captures it all.
However, since the rumor is that we will still be able to use Tower (DroidPlanner3), then its stands to reason we will be able to connect to the Solo via a Mavlink radio and use Tower's more Action Sports "friendly" Follow-me functionality. Lets hope so.
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I had the same question, called 3DR and was told that the follow me function would require the controller stay within 100 or 200 meters of the mobile device being "followed" by solo. Testing was still under way to determine safe distance.
Per the manual:
If the Follow subject is engaged in an activity that prevents them from using the controller, you are required to use a safety pilot during Follow. The subject will carry the mobile device and be followed by Solo, and the safety pilot will hold the controller and be ready to regain manual control at any time.
That bites!!!