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: Discourse Meta
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”.
See also:
我有大约40名信任等级为1的成员。群组可见性已设置为“所有人”，但他们无法看到彼此。这是为什么？
这是一个自动组——自动组与手动组的规则和行为不同。