Spaces can either be 'Public' or 'Private'.
By default, Spaces are 'public' on creation. This means that any team member with a Wake login will be able to see posts in this Space.
To restrict visibility, you can create a 'Private' space, which allows you to limit access to only specific users.
Once a Space has been created, you can adjust whether it is public or private, as well as add additional users to private spaces in the Spaces settings menu.