XF 1.5 Commenting on posts. Stock or Add-on?


I'm running a small board and the Xenforo board we migrated from gave members the ability to comment on a post:

Topic: Baseball

New York Yankees Players (thread)
Babe Ruth (post)
Lou Gehrig (post)
Gehrig's farewell speech made me cry! (comment)
Me too! (comment)
I've looked through the Admin options and such and I couldn't find anything, so I'm guessing it's an Add-on.

If not, please point me in the direction to turn commenting on. Such a basic feature I figured would be standard.

If an Add-on...any recommendations?

Thanks in advance!