there’s a setting on the groups page; “Who can see this group”; if I set that to public; my group messages will still be hidden right?
Yes the category security is something different.
All this means is that people can see the group exists on the group page here: https://meta.discourse.org/groups
They can also see who is a member of it etc.
It might be worth adding that the setting “Everyone” has a different meaning depending on whether the forum is public or private. In public forums “Everyone” basically means “the whole world”. In a private forum (i.e. which require users to login to see anything), it means “all logged in users”.