Hi there, and welcome to the forum.
1) To create multiple user profiles, go to Settings > System> Users, and you will see the option to add a new user.
If you're just coming to GrapheneOS, I recommend keeping it simple and not bothering with multiple profiles to start with.
2) It works the same way it would on Android. Settings > Apps > See all apps. You can then find the app you want to adjust permissions for and tap on "Permissions".
Keep in mind that most apps will just prompt you for permissions when they need them as you're using them.
3) To install sandboxed Google Play and use the Play Store, on your home screen tap on "App Store". Then, tap on "Google Play Services". Tap Install. Wait for everything to install (it can take a while for everything to be installed and you'll be asked to confirm the installation). You can then find the Play Store app installed and login to your Google account and start downloading apps as you would on regular Android.