If I look at this first, just from the standpoint of me sending messages and getting responses everyone can see:
1) Use the notification feature in Android, to show there are messages pending for them to read in Crew-Mgr
2) The ability to see who has read their messages, vs have not, and watch as it flips from not-read to read. leave an icon to click on phone number, so I can call people who have not read a critical message. "we are meeting at 9pm instead", 'the race tomorrow is canceled", "can anyone volunteer to bring xxxxxxxxx tomorrow"
3) Know that people have not given permission in their smartphone to receive crew-manager notifications, and push a button to request that they do so with a link to instructions on how to do it in Android and Apple.
Are you aware of any other application that has a feature like this that can be looked at for possible integration? This is not trivial to code.