"Are you talking with your friends?"
Translation:Czy rozmawiasz z przyjaciółmi?
How common is it to leave out "twoimi"/"waszymi" from phrases like this? The implication that it's specifically your friends is of course obvious in this particular sentance, but is there any formality lost by not including it?
Very common. It's just redundant to use it, if they are friends of the subject of the sentence. It is then logically assumed that these are the subject's friends. Formality... no, I wouldn't say so.