by default it seems the tagger puts it next to the artist.
By default the tagger puts it where the -source- decided to put it.
I believe it's really a user's personal preference. In the past I always place it as part of the title such as "Title (feat. some artist)."
But I discovered that makes it difficult to do a search for the featured artist.
With some help from boroda and his amazing Advanced Search & Replace plugin module, I copied all the featured artists from each track to Guest Artist. I also left the featured artist as part of the title. 400+ files took a few minutes once I was satisfied it was working as expected.
do you use the featuring tag inside artist tag or in the separate guest artist field?
Personally it's not part of the Artist. It's part of the song. So if you're going to make it part of something, make it part of the Title.
is there even an automated way to put the feats in their own box?
Use <Guest Artist> tag and the Advanced Search & Replace module from the Advanced Tagging & Reporting Tools plugin. This assumes you already have a "featured artist" in the track title or album title tag.