There are several things to mention here.
Social Attitude does not equate directly to cognitive function introversion or extroversion. On a cognitive function level, introversion means 'micro focus' or focus on the small scale and details. On a cognitive function level, extroversion means 'macro focus' or focus on the big picture and how things relate. Therefore people with strong extroverted functions often have a focus on groups, while people with strong introverted functions often have a focus on individuals with respect to social interaction.
The degree to which someone feels compelled to express themselves or their personal level of shyness or even social inhibition may or may not indicate cognitive introversion or extroversion. It is commonly the case that these things correlate, but by no means a rule. There are plenty of gregarious introverts (who will talk one person's leg off) and there are also extroverts that don't talk much (but love being in groups).
That said, as people grow and develop, they begin to develop all of their cognitive functions. For INTJs, those would be Ni, Te, Fi, and Se. While we develop and use these functions at all times to varying degrees, we tend to favor our lead function most, our secondary function a little less, then our tertiary less than that, then our inferior least. This causes us to develop our dominant function to a fully operating capacity by around the time we hit our teens. At this point, we begin to see the biggest gains in our secondary function, until around our late teens for most people. Once our secondary function reaches solid operating capacity, we begin to see gains in our tertiary function until around our mid 20s for most people. Around that time, we go through our last push and develop our inferior function until around our mid 30s. This is one of the reasons that people are generally considered 'mature' around that age, because at that point, they've developed their functions well enough to deal with most situations and relate to most people on some level.
For a typical INTJ, this would mean Ni is developed most through ages 0-18. Te is developed most through ages 5-23. Fi is developed most during ages 10-28. Se really blossoms through ages 15-38. (note these age ranges are extremely variable to individuals, their circumstances, influences, and motivations)
Therefore, if an INTJ was going through a phase were their Te was really starting to come into its own (typically late teens to mid 20s), that person will experience a strong sense of expansiveness in their extroversion. They'll also get another push toward extroversion when their Se finally kicks in (typically late 20s to late 30s) but it will be a more 'maturing' and 'balancing' effect on their personality.
Will this make someone more likely to engage the world socially? Absolutely. That macro focus is expanding to include more and more people in any given setting. You're better and better equipped to deal with larger crowds for longer periods because you're less and less overwhelmed by all the details. Your extroverted functions have begun to do their job - which is acting as a buffer against the barrage of details that your introverted functions are picking up on, which allows your introverted functions to focus on the details they want to - thus conserving mental energy.
The more you develop all of your functions, the better equipped you are to deal with the world.
How do you become extroverted within the range available to an introvert?
Engage your Te and Se more in solo activities like working out, getting out in the sun, mental exercises, playing chess, first person shooters and real time strategy games against other human players, etc. This will charge your Te and /or Se, which will amp your interest in engaging the world on a macro scale.
Always remember this: Introverts have to deliberately engage their extroverted functions.
If we don't put our extroverted functions into motion, they will eventually weaken until they reach an inert point. Our introverted functions will always operate whether we want them to or not, but we have to keep adding energy to our extroverted functions. The more you put into them, the easier it is to keep them in motion - much like an inertia effect.
I have the inpression that when I used to use my Se a lot (e.g. photography) I also used to be more extroverted.
Surprisingly, there is a lot of Fi in that activity. However, Se is obviously being used. As an Enneagram 8, you're going to have a strong Se. We tend to be hypervigilant and alert to the world that is "a hostile place". If you've been feeling safe and content, as an Enneagram 8, you'll begin to slack off from engaging your Se because you won't see so much of a need for it. And, as an introvert, your extroverted functions aren't self sustaining unto themselves.
Solution: Force yourself out of your comfort zone and activate those Enneagram 8 active defenses. Engage challenges. Put yourself in a position where you reflexively feel the need to project strength. That will amp your Se almost instantly - like riding a bike.
I can only explain this with my being an INTJ and my Se temporarily becoming a tertiary, and perhaps temporarily becoming an ENTJ.
I used to think that as well, but now I'm very convinced it's just the Se mechanism of Enneagram 8.
