I guess what you are trying to say is "ads" and not "add",  but the link given above by @DireWolfM14 explains it all, because from the link, I was able to capture a statement given as below,


"In addition to the inhouse forum ads, it is common for forum users to sell ad space in their signatures. These ads are not intermediated by bitcointalk.org in any way. If you are interested in advertising in this way, it is recommended that you hire a signature manager."

But, if you happen to be new into signature campaign, here is a brief explanation. Signature Campaign is a kind of campaign just like ads, whereby members of the forum are hired to promote a given project by means through the means of post count and been paid a given $ rate either weekly or monthly.

But for a signature campaign to be successful, 3 things are involved.
1. You have a hire a signature designer who will design a very good looking signature for your company
2.  Hire a manager who will manage your signature campaign project.
3. Hire a specific forum members base on rank to help promote your project on the forum

And you can get a good Signature designer and Manager both on the "Service" board of the forum