People who J.I. usually have no friends, otherwise they'd have conversations/discussions of their own with friends of their own.
This word was started at Auckland Grammar School in Auckland, by the 2003 class of 3G. Or so they say. The word "J.I." has reached other schools in the Auckland region (and most likely) other parts of NZ.
Related to tag along.
Verb: "Hey my bus will be delayed by at least an hour so can I J. I. with you guys?"