It can
make the introductory text is visible from all but the full text by
registered users?

This may be an answer to what you want
community.getk2.org/forum/topics/introtext-public-full-text?commentId=3536014%3AComment%3A53962

probably a better way, i just came across.
Edit item.php in your template override
use code similar to what you find at
www.gws-desk.com/tips-tricks/137-hiding-template-positiuon-with-logged-in-user
Where the fulltext is displayed wrap it in an if statement that checks wether logged in or not