Hello hjmf1954,
Advanced Data Protection encrypts you data store within iCloud. This includes:
- Messages Backup
- Photos
- Notes
- iCloud Drive
and more.

You choosing to turn on Advanced Data Protection for iCloud has no impact on members of your Family Share group. As someone in the security field, I highly recommend turning this on and having your Family Share members do the same. It will not impact anyone and you won't notice any differences. All it does is encrypt data that you have stored in iCloud that wasn't encrypted before.
How to turn on Advanced Data Protection for iCloud - Apple Support.
It will turn off access to iCloud.com by default. I recommend keeping this off, it prevents anyone from accessing your data because they would need one of your devices to access it. However, if you use iCloud.com:
- Open Settings
- Tap Your Name (Apple ID)
- Scroll to the bottom toggle on Access iCloud Data on the Web
This can be toggled on/off at any time. My personally recommendation if you don't use iCloud.com is obviously to keep it off permanently. However if you do use iCloud.com, I'd recommend keeping this off until you need to use iCloud.com. Once you need it, toggle it on and you'll be able to use iCloud.com. Once you're done with it, toggle it back off.