Charlotte's angels is the name Ellen used for her and Nanase when Charlotte mentioned that she likes knowing she has a couple of angels to call on if she suspects spooky things appearing. The "angel" part is a reference to their Guardian form.
Their first case took up most of the Family tree story line. Rhea had invited Nanase and Ellen, due to their career as Meddling Teenagers, to a party with college students. Her sister, Diane, and her roommate, Charlotte, were also present, and those four were the only ones not affected when Charlotte's "acquaintance" from a forum, Not-Tengu, attacked the party with his magic -- channeled through the spiked punch, which they hadn't partaken of. As they searched for a place with good enough mobile connectivity to call Edward Verres for help, he then attacked them directly, whereupon Nanase used her Guardian form and Ellen copied it.
Their second case happened in So a Date at the Mall on the same day as Elliot's first date with Ashley. Charlotte called Nanase and Ellen due to some mysterious figure's appearances and they brought Diane along as a driver and because she wanted to help solving a mystery.
The name is a reference to Charlie's Angels, a group of three beautiful female private investigators with no known magic powers (nor wings), although they at least know martial arts.