By default, your community members can leave comments on your campaigns.
If you don't have this activated in your community right now, and you would like to activate it, you can do this via settings > actions:
Two elements of the comments can help you to increase the engagement on the platform:
1) Members can mention each other in comments by typing @ and then the name:
2) Members can leave replies to comments:
Don't forget to enable workflows in the settings if you want to receive email notifications when members leave comments, or when you are mentioned in a comment.