In the mean-time there are work-arounds. You can invite the person to your fireteam, that will enable voice chat (provided the other person is using their mic). The other option - send a friend request to the person, next start a chat party and invite your new friend to your party. Also in DeeJ's latest post he stated that chat amongst fireteam members (even randomly generated ones in strikes and crucible) should be getting chat in the near future. Proximity chat does not sound like it's coming anytime soon.