I dont think just "jumping in" works that well. Lots of weird situations when the odd hippie from the corner who never talks just pops in during a convo. But the thing thats made me more social lately, besides unnatural use of alcohol, has been training Ne.
Every INTP knows how to do it, on the occasion when we feel secure were basically ENTP's. We dont need "training" on how to use Ne, but kind of when to use it. I realized that when this ENTJ made sarcastic remarks on everything my teacher said on a psychology class, while i sat comfortably hazed in my Ti. I was feeding my Ti by thinking of something completely useless, while the ENTJ got his satisfaction by the outside stimuli.
Every INTP has social connections, okay not that sure, but the point. Every INTP knows the strange satisfaction you get from using Ne and making people giggle, its like heroine injected to your frontal lobe. BUT we just dont use Ne as often as ENTP's, so in my opinion the first step is trying to force yourself out of that comfortable hellish gorge of Ti, and SEEK for situations which make using Ne possible, and the biggest point is getting that satisfaction out of it. Makes you want to have more. See whats around, everything can be turned into something beautifully irrelevant and silly when using Ne. Just socializing doesnt make you an E, consciously increasing the usage of the second function makes you an E, I's can be social too, right?
And my experiences with my adventure into the tempting world of ENTP'ness? I've got kicked in the head by an ESTJ woman who apparently really appreciated my sense of humour, and people have started calling me insensitive, dunno why. I also have a flu, not sure how that's connected but i remain suspicious.