If you invite Guest users from other tenants to your Power App it would be very easy if they could change the organisation in the Power App. If they login to power apps whit their account the can only see the Power Apps in their own organisation. They can only see the Power App form the guest organisation by opening the URL of the Power App from the guest organisation. Power Apps shows "changing organisation" and you see the App from the guest organisation. Since mobile phones in an enterprise world are more and more protected (f.i. Xenmobile), the Power Apps url does not start the Power App in such an environment, but instead opens it in a browser where you receive cross site cookie errors. So starting the guest power app through a url doesn't work in these scenarios. You can work around this by building a simple Power App with only one Button which launches the Guest Power App by using the URL scheme: ms-apps:///providers/Microsoft.PowerApps/apps/1111111-1111-1111-b4b4-ebbbd7ba2001?tenantId=111111-1112-1111-a112-cf2f18f8aa53. If you start this simple app (use a url shortener like bit.ly tot make it easier) from Safari on a iPhone, you can start the Power Apps app in the guest organisation. The user has to login twice, not very user friendly, so please build an organisation change option into the Power App!