Comment by Cthulhu_

A simple rule of thumb is that people should mind their own business, i.e. if it doesn't affect you / your day, leave people alone. If someone chooses to browse their phone during a meetup (which I presume you refer to), that's their loss; while it's rude that someone doesn't seem to pay attention to a speaker, it doesn't affect the speaker or the rest of the audience unless they choose for themselves to make a problem of it.