It is possible that the Administrator of your account (responsible for Communication, Human Resources, or General Management) has created a read-only group.
This type of group is a tool that Ommnio makes available to Administrators for one-way communications. Messages are sent in them, but it is not possible to reply. The main reason why an Administrator creates a read-only group is to avoid the chaos of responses from participants (e.g. if it is a group with a lot of participants).