Went through the agony of this myself for multiple users. First make sure the other phone is logged in with the same email that was used to invite them in a newly installed ST app. Don’t try to install another hub or anthing else.
Chose the three bar icon in the upper left. Then note the home location at the top of the drop-down menu. Then Within that click the small drop-down arrow and then click the shared location at the bottom of the list.
You will now have access to all of the shared devices, etc. While on the secondary ST app, click to add a device and then a presence sensor for this mobile phone.
Make sure you have shared location services with ST app and disabled battery saving so it will run in the background. Presence for this phone is then based on it’s location service reporting in relation to the definition of the shared location in the master ST app.