Ok my share of assumptions now
Gauri is hell-crazy and possessive when it comes to Gauti
Gauti may have liked Neha but she considered him just a friend
He may have asked Gauri to bully Neha with Paanch so that he could rescue her and impress her. Maybe even the kiss ragging was pre-planned!!!
Gauri did that but Neha's spirits were unputdownable. Paanch developed their own grudges against Neha due to her audacity and things got worse.
Gauti decided if he can't get Neha "this" way, he will get her "that" way by drugging her with Paanch's help and Neha dies as an accident or commits suicide.
Yudi may have a grudge with Gauti for his sister or cz Gauri is way too possessive about him or they may have had an argument amidst all the Neha hullabaloo.
Phew... That was too much for one go, but its what I think of the future track!!!