I have had the Kidnappers quest (from Modest Inn) twice in this game. Both times I went to the Wildlings camp and fought for the captives.
Both times it said they would join me as pioneers. Neither time did I see any new Pioneers. The first time my army was full so I thought that was the reason, but the second time I had 2 vacant spots & still didn't get any pioneers.
Here a saved game prior to completing the quest for the 2nd time.
https://www.dropbox.com/s/hffmi5vqsh55poj/100_63.EleSav