This is necessary if you want two people to develop the same map android app.
Once you have a maps api key (see a previous how-to) you'll want to share your debug keystore with everyone else working on the app. Otherwise, the maps will just show up as a gray box with grid-lines.
You can find where your debug keystore is if you're in Eclipse by going to Window -> Preferences -> Expand 'Android' along the left -> Build. You can see the path to the default debug keystore. Copy the keystore and give it to your teammates.
To use a keystore other than the default (i.e. use the keystore your teammate gave you), first save your teammate's keystore. Then go to Window -> Preferences -> Expand 'Android' along the left -> Build, and override the custom debug keystore to be the path to the keystore you saved.
How to: >