How informal can 为 someone 服务 be？
This totally works colloquially.

One of the most famous slogans in Chinese is: 为人民服务 (Serve the people!) - there is not a single Chinese person who doesn't know this.

Likewise you can always 为 XXX 服务, this should be very easily understood by almost anyone.

edit: maybe it would just sound a little pedantic...you wouldn't exactly say that there's two people serving you in English, though, would you...