I would love to be able to refer a client to another guide on days when I am already booked. I currently do this by texting other guides for their availability and if there is a match I get 25% or the booking fee.
Also give the guide an ability to add other guides as "Connections" or "Collaborators" so that they can see each others calendar availability. This will also allow for a more fluid referral process and a better outfitter or fly shop experience.