By default all users can access all Personal Spaces. If access to a Personal Space should be limited, each user can apply page-level restrictions to their Personal Space.

Could the default access be defined by membership to a Groups?

For example, if a user belongs to the same Group as me, then they can access my Personal Space. Otherwise, they can only access my Profile.

User can still apply page-level restrictions to their Personal Space if additional access is to be granted/restricted.