Spanish, unlike English, (usually) has a different verb form/declination for the second person of singular and plural. You are right, in English if I say "How are you" when a group of people is involved could be unclear if I'm addressing the group or just one person. That's why is necessary to use "you guys" to emphasize that you want to address the group.
On the other hand, this will automatically understood in Spanish by the usage of the verb form or pronoun
¿Qué tal estás? ¿Qué te parece? ¿Quieres un caramelo?
¿Qué tal estáis? ¿Qué os parece? ¿Queréis un caramelo?
So, yes, you could add something that emphasizes the group you address. It is not necessarily redundant, but is not necessary either to point the difference.
¿Qué tal estáis todos? ¿Qué os parece, gente? ¿Queréis un caramelo niños/chicos?
Remember that Spanish favors the usage of masculine to address a group with people of both genders.
Even if you use the "usted" form (which basically uses the third person verb declination) you are using a different form for singular and plural
¿Qué tal están ustedes? vs. ¿Qué tal está usted?
If there was a verb that had the same form for singular and plural (I can't think of one right now, but that doesn't mean it does not exist) you will then of course want to clarify by using the word that addresses the group.
Maybe the usage of voseo (countries like Argentina) could be that exception. The differentiation between singular and plural is not always clear (even if the use of vos is as a second person singular pronoun and usually a different for is used for plural). For example (see this reference) the voseo of Panama and Venezuela (and Southeastern Cuba and Northeastern Colombia), unlike the "standard" voseo", can't distinguish from the 2nd person plural of Castillian. Still, some of these countries still use a different form for plural than singular, but could lead to confusion in written Spanish, without enough context, or maybe when addressing a group of people who speak different variations of the language.
Some possible options would be
- Señores, caballeros ("¿Qué tal están, señores?")
- Señoras, señoritas (idem as above, to a group of all females)
- Niños, chicos, muchachos, etc...
- Gente (an informal way to address your group of friends)
- or even "todos" or "cada uno" (¿Qué os parece a todos la idea? ¿Qué pensáis todos (o cada uno) de la película?)