When you first download Bloc your settings will be set to public. This means that if you create a Moment without changing your settings, everyone who follows you will see your Moments.
You can set your profile to private, which means people need to request to follow you. If you accept them, they will be able to see your Moments.
When you create your Moment, there is an option to only show this Moment to your ‘Crew’. Your ‘Crew’ is a list of people you have decided to be part of your close group network. If you select them when creating a Moment, then only they will be able to see it.