Multiple workshop users

I have an app that I need to use at a workshop of 30 users. It is relatively small and deployed on the shinyapps.io on a standard plan. So it should be okay....(?)
I trialled this however with 15 users and it greyed out for them all - couldn't connect.
Is there something I need to tweak in the settings? I have tried to read the material on workers and instances but I admit I'm unsure as to what to tweak
Also, is there an issue with having all users on the same wifi?
Thanks!