From the Web platform, Managers can pin a post to your group once you've sent your post to the whole group. To pin a post to your group, click the drop-down menu on the right-hand-side of your post:
Choose Pin post. Once chosen, your message will be pinned to the top of your group. Each group you have can have its own unique pinned post.
Similarly from the mobile app, clicking on the post options menu, selecting Pin Post will pin the post in the group.
Currently, only one post can be pinned at a time.