It's actually VERY easy,  

 
Simply go to any post, take mine for instance, look at the right side of the post at the bottom and you will see a drop down menu labeled "Actions" Click on the option "ignore user" that will bring up a list of ALL of the posters you have on ignore, simply hit the "x" on each member that you would like to "un-ignore" and it's as easy as that.

 
Welcome back btw.

There’s almost always multiple ways to do things on TER.  For your question, GaG’s reply is one way. Another way is to click on “messages” at the top of the page, then on “ignore list.”  
You can remove or add people there.