The sidesaddle goes back to the 13th Century, but the ones made during the 16th Century were more accommodating.
In the 1830s, Jules Pellier invented a sidesaddle design with a second, lower pommel to the sidesaddle.
The impact of the second pommel was revolutionary, with the additional horn giving women both increased security and additional freedom of movement when riding sidesaddle allowing them to stay on at a gallop and even to jump fences while fox hunting and show jumping.
With this design, nearly all recreational equestrian pursuits were opened to women, yet they could also conform to expectations of modesty.
Women wore a 'riding habit' - similar to clothing worn in everyday life, but specifically designed for riding sidesaddle - the first being developed in the 16th Century.
In 1875, the first safety skirt was introduced and later evolved into the open-sided apron.
By the early 20th Century, it became socially acceptable for women to ride astride while wearing split skirts, and eventually breeches - at that time, the sidesaddle fell out of general use for several decades, but it's use has been revived as a historical style of riding.
Given how they're dressed, the photo would date from that later time frame, and given the number of them, there's a chance that they're rodeo girls of the era.
There are several books on that topic and the riders alone that make for interesting reading.
Scouts Out!