CRJ reserve FOs had a GroupMe thing set up for reserve day swaps, trades and such. The etiquette called for contacting the FO whose flying you are interested in appropriating, and then proceed with the other party's blessing. Or not. Guys would also advise of the assigned trips they were willing to part with in the same group. Worked pretty well, but the overall pilot group was small enough to where it was fairly personal, and constant over the months due to the lack of movement.