"llamen" is the third person plural form of the imperative (same as subjunctive) used when addressing more than one person http://conjugator.reverso.net/conjugation-spanish-verb-llamar.html
The obect of llamar almost always takes "a", even when not a person. Ejemplos:
Llamo al hotel.
Me gusta llamar a esta idea «socialismo creativo»
Debemos llamar a las cosas por su verdadero nombre.
One exception is the phrase: Llamar la atención (to draw attention )
Llamamos la atención hacia estos asuntos
When you call by phone or to someone, you should use "llamar a." Some people explain that that is a personal "a," even when you're calling a building (e.g., hotel, hospital, school, etc.) because you're not really calling the building, but you're calling the people in the building.
Other people say that it's to clarify that you are calling via phone as opposed to calling someone/something a name. For example, if you were writing that you called the police or the hospital and you didn't use the "a," there may be confusion. Did you call to the police or did you call them some name (like "the men in blue)? Did you call to the hospital or did you give it some name (like "the Children's Hospital")?
Hope that helps.
That's probably too informal for duolingo. You can be either singular or plural. The "guys" isn't necessary.
I have a British friend who objects to "you guys" and has taken to calling everyone "you ladies." Her reasoning is that if "you guys" can be non-gender specific, then so can "you ladies."