You can only lock a post if you're a Group Owner or Admin.
Here's how to lock a post in a group:
1. Click on the post title.
2. Scroll down to the bottom of the post.
3. There you will see four buttons ("Delete Post & All Replies", "Lock Post", "Move Post" and "Feature Post") and a drop down menu ("Post Status").
4. Click the "Lock Post" button to lock the post.