Note that unlike most examples in this lesson this is not the present progressive tense which follows the structure of "estar (conjugated) + gerundio". It is however apparently something pretty similar. "seguir", "continuar", "andar", " ir" and "venir" are all frequently used in conjugated form before the gerundio. See http://spanish.about.com/od/verbs/a/verb_gerund.htm
'You are still eating' is much more likely to be said than 'you continue to eat'! "We can't leave yet because he is still eating" is a good English sentence, immediately understood and normal-sounding. "We can't leave because you continue to eat" sounds odd to any English-speaking person! I can think of only a few contexts when the sentence would sound okay like, "I have asked you to stop eating that ice cream yet you continue to eat it, so I'll have to take it off you." In this sentence there is the extra level of meaning i.e. "You insist on eating even though I have told you to stop."
Me to. Most dictionaries simply translate "ustedes" as "you". Only google es>en translates it to "you guys" , "you all" and even "they" . BUT in the DUK (DisUnited Kingdom ...GB) we more probably would say "You lot continue studying" (while i pop out for some air), when we are referring to more than one person. Also "You guys" is a U.S expression which has become gender neutral. Remember "guys and girls"?
The worldwide standard is that the English pronoun "you" is both singular AND plural. Context alone is what helps you understand whether the singular or plural meaning applies. "You all" is reserved for certain dialects, oral speech, and literary characters. In fact, using "you all" is considered irregular in scholastic, legal, and formal writing.