Two things.
Why can’t I send text alerts to more than one phone number for the same event?
…and why do I have to key in the phone number every time? If I am registering phones through the app, why doesn’t the app know the phone numbers and just let me select a given phone? I should be able to manually add any number, but it should know the registered phones already.
Either I’m missing the functionality, or you have a silly oversight. Not everyone is single! 
They used to have a contact book . Then it went away. Then some people found some hacks that made it work but that could break notifications in SHM. I think now it’s just gone.
Not sure why, it would obviously be a valuable feature.
You also used to be able to contact multiple numbers with a text by using the SMS IFTTT channel. Now they’ve changed that feature so it will only send to the phone associated with your IFTTT account. I think it’s also limited in the number of text can be sent each month. So that method that no longer works for most people . 
still works for me on two diffferent hubs.
It is ridiculous that you can’t add more than one phone number. I am talking to you SmartThings engineers, please fix that.
They probably don’t monitor the forum, do they?