Jet Airways withdrew the access for a +1 guest for higher end elites in 2011. Before 2012, the policy for lounge access used to allow JP Platinum members to bring 1 guest along to the lounge, regardless of the fact if the Platinum member was flying Economy or Business class. As a not so frequent flyer then, I used to benefit travelling with my colleagues at that point of time who were Platinum tier members.
However, with effect from February 16, 2015, Jet Airways has reinstated this benefit for Platinum tier members. Which means, Jet Airways Platinum tier members can now help a family member/friend/colleague into the lounge along with them as well.
Do remember however, that this lounge access is conditional to the fact that the co-passenger being guested to the lounge should be on the same flight as your own. You need to be booked on a Jet Airways marketed and operated flight, i.e., the ticket number should start with 589- and the flight number should be with 9W.
Lounge access would be valid at domestic and international airports, where Jet Airways offers lounge access to their passengers.