First time user and have App?

Dumb question here but new to this. Looking to use this with my students due to COVID-19. On the app it says server URL what does that mean?

Also, the only way I can invite my students on the app onthe phone is by typing in their phone number? Is there an email link that can be sent to them?

There is an info button with a copy link which copies the invitation that you can use in your email.