Under the "groups" tab in the chat, you'll find all the groups your are currently member of. No matter if it's a public or a hidden group, only members of a group can participate in the group chat, or even read it.
New group members can only see messages that were posted after they joined. There is also an automatic notification in the group chat, whenever someone new joins the group.
Messages posted in public groups count for gamification matters, but conversations in hidden groups do not. The latter behave like private conversations, gameplay wise.