Our company has subscribers and then we have people who are interested in subscribing. For the people who are interested in subscribing, I don't want them to see all the subscriber messages, but if they do post, I'd like for subscribers to be able to respond. So, in the end, I want them to participate, just not have access to all of the subscriber conversations.
?I think the only way to have them unable to use the Messages page to read messages is to not subscribe them to the group. Then you can check
Allow Non Subscribers to Post
in the Message Policies section of your group's Settings page. This will force moderation on all of their postings, a necessary precaution to keep out spam from non-subscribers.
Then you would also want to select Sender or Group and Sender in the Reply To control so that member replies go back to them. Unfortunately that will also affect members' replies to each other. I don't see a way around that.
The other option, if it would be acceptable for these "type 2" subscribers to see each other's messages, would be to include them in the primary group, but not in a subgroup devoted to the type 1 subscribers. The type 1 subscribers could converse privately in the subgroup, and the type 1 subscribers could answer type 2 posts in the primary group.
That would be a little like the "member" versus "board" structure I have in my PTA group, where all PTA members are members of the primary group, but only the elected board members are members of the "board" subgroup.