It is a line from the C&W song “Beer for my Horses”-usually, a lead cowboy would just say “saddle up”, but saddle up your boys means the same-saddle and get on your horse, it is time to ride...
Blame the vagaries of English for making this difficult to express in the context. “Get your boys to saddle up” isn’t as laconic. Cowboys don’t get garrulous, there isn’t time. Yee haw.