How to Post Message in Google+ Community
Whenever you share information and updates about your business in the community, all the members of that community will get to know about your business. To post your message click on the community and click on the section (what do you want to share?) located in the middle. A box will pop up, write message and click on the "POST" button at the bottom. You can also add photos, links, etc, to your post.
Pick a suitable category by clicking on it. Your message will be posted.