How good a swimmer do you need to be to join masters?

How good a swimmer do you need to be to join masters? (And to fit in reasonably well, or at least not stick out like a sore thumb! :D ) I know I could never keep up with a masters group now, but it would be nice to have a goal to work toward. Do you need to swim all four strokes? How about stamina/endurance/speed? Can anyone give me an idea of what a baseline level would be?