In spoken language, I'd say that just "Buen fin de semana!" is more usual, addressing either to one person or to a group.
"I appreciate it": "Te lo agradezco" if you are talking to a friend, a family member, etc. Formal: "Se lo agradezco", you should use it when speaking with someone you wouldn't address by his/her first name in English. Plural is "Se los agradezco" in both cases in Latin America. In Spain, "Os lo agradezco". If you feel confused about which one to pick, a plain "Gracias" or "Muchas gracias" will do the job in all the cases.
