Before you can use a project that is managed by another organization, you need to accept the organization's invitation to join their project's group. See Join groups in the ArcGIS Online help for details.
Some of my mobile workers haven't moved in a while, but they told me they are no longer at the location that is marked on the map. Why aren't their locations updating?
There are a couple of scenarios in which you might see this. One is when the mobile worker doesn't have a network connection: updating their location requires that they are connected. It could also be related to the mobile worker's status: the mobile worker's location is only updated while their status is Working. When the mobile worker's status is On Break or Not Working, their location is not updated on the map. The mobile worker's location could also be out of date if the mobile worker is using an iPhone or iPad and didn't set their status to Not Working before the app was shut down.