As far as getting keys to other sitters on your team there are a number of options. Personally I keep a box outside my front door. When sitters need a key, I just put the key in the box and assign them a key pickup using the key management tools within my pet sitting software.

I have heard of some sitters in larger cities renting mailboxes around the city and transferring keys through a mail box. 

The box that I recommend using is available in a link in the description below.