1) If you are on the same Local Area Network that is connected to the internet, and are both using Wii U, yes, you can both play with 2 other online players. If the Local Area Network is not on the internet, you cannot play with anyone. Just each your own game on your Wii U. You need to both connect to the Nintendo servers that are online.
2) If one of you is using the Nintendo 3DS, he can connect to the WiiU and then both of you can play with 2 other online players, for a total of 4 online players. As long as your Local Area Network is connected to the internet.
3), If both of you bring one of the helpers (Cha-Cha or the other), you cannot ask anyone else to join in your hunt. A total of 4 characters can be in a hunt at a time, and Cha-cha and his buddy each count as 1.
Hope that answers your question.