Playing around on the GR site a bit, it's not straightforward how to find authors to follow. You pretty much have to know you want to...
I kind of assumed that's how you used FactFict. (Never having used it myself.). I expected that when you found a series you liked, you went to FactFict, searched for it and then clicked "follow" or something. Did you browse there looking for books?
...then have to find a listing for one of their books and then click the author name. Then you can follow them. Even if you don't it looks like it does provide a listing of books, and also shows what series they have. I looked up an author who I know has more than one series and, sure enough, they're both listed on the author page. Followed a couple .... will see if I get bombarded with promos. 
I did a search for "In Death Series" on GR, then clicked on the author name. Also searched by the author name. Wasn't difficult. That seems pretty much how one would do it on any site? Again, I don't recall ever using FactFict more than maybe once, years ago, so I really have no idea how it worked.
I also signed up for a couple of authors. We'll see. We should also send a request into GR to allow us to follow series.
Betsy