There's a few actually.
The simplest one would be a person "opening up", which goes equally for any individual trying to become more sociable. You would say "that person has really opened up!" But mind that it can also mean opening up to new ideas, rather than being exclusively social.
Another good one is "coming out", which is usually paired with "coming out of his/her shell" or "coming out if his/her cocoon". But be careful with this, because it can also mean "coming out" as a homosexual (usually paired with "Coming out of the closet"). If you pair it with the shell/cocoon metaphor though, it should be clear what you are trying to say.
To clarify, these are more permanent concepts. You may be looking for a more temporary or one-off event than I have described here.