If you already have a Landscape username and need to be added to another (maybe you volunteer with multiple organizations), all the other organization needs to do is add you as a user. Once they've done so, you can choose which account you interact with via the drop down menu in the Profile.
Accessing Multiple Accounts with the Mobile App
In order to switch accounts with the Landscape Mobile App, you'll first want to make sure that any past site visits have synchronized by selecting the sync button. You can then change accounts on the desktop as instructed above. Sign out of the mobile app, then back in again, for the information to be updated.