If you are using an adjective to describe a pronoun in Russian, and that pronoun describes a group containing both men and women, what form of the adjective should you use (masculine, feminine or neuter)?

For example, how would you say "They are tired" in Russian where "they" = both males and females.

Also, if you are using an adjective to describe "you" (singular), but you don't know the gender of the person you are describing, what adjective ending should you use?