Starting from Peepso 1.10.4 you can set your groups into read-only mode. Which is mean, only owner and site administrator who has privileged to create new post. Group's admins, members they cannot post anything. They just can read only the posts shared by owner. It's just like announcement group, where you need your members to pay more attention to the informations you shared.
To activate this setting, go to Frontend > Your Group Page > Settings There is a new setting "Disable new posts", set to yes if you want to disable new post for members.