First of all, it looks great, good job! My question is that why do you limit yourself with only fitness? For example, study sessions are a great example that can benefit from your product.
Basically it was the use case for me personally. I had to spend 2 weeks on quarantine totally alone separated even from wife so we wanted to exercise together. But yeah, I think you are right, there are many potential use cases!
First thing that came to my mind too! Last night I watched a YouTube video with a friend, and we did a manual 5,4,3,2,1 countdown to synch, and then text chatted via WhatsApp, video chat of one's choosing could also be used. But rolling it all into one is nice.
It would be nice to be able to do the same thing with a shared Plex server, but not sure if that would be as easy.