At a Hogwarts themed birthday party for my DD, I taped a sign across the double-door'd office (effectively sealing it), with an arrow pointing left. The sign said, "Moaning Myrtle" (The kids got a kick out of it, and it kept them out of my office, too!)
If I were hosting any other group, I'd do something similar, as the washroom is around the corner out of sight from the main part of my house. Closing doors to rooms meant to stay private, leaving the washroom door open, and a light on inside makes it clear where guests should go. Signage just helps point the way.
I agree with all who've said, if you know where the bathroom is, unless guests are all seated together (i.e. dinner table), it's fine to just go use it. If seated together, excuse yourself quietly and go use it. If you don't know where it is, quietly ask the host where it is, say thank you, and go use it.