Both you and your friends should have 8.1 installed and wifi sense activated, you must have the number of each other in the phonebook. Microsoft says the thing may require some days to register and let your friends use your wifi. Oh and check if the wifi is shared in the managing page
I gave this now several days, basically since my last post and nothing happened yet. I followed all instructions online given here. Well, let's hope that this is a DP bug that got fixed.