Is introvert a derogatory term? Does it have the potential to offend someone, or is it okay to call someone who enjoys solitude an introvert?
A long-winded discussion on introversion and extraversion is way beyond the scope of this website; however, generally speaking, introverted people tend to avoid big crowds and enjoy solitary activities.
The word in itself has no explicit negative connotation associated with it, as it's simply considered a type of personality by professionals like psychologists. That being said, however, in especially Western societies, introversion is often stigmatized and stereotyped by others, such as in the media and is generally (incorrectly) considered undesirable.
There is a potential for offense, depending on how the comment is taken. However, this is more to do with perception than the actual term. I would happily describe myself as an introvert, because it is true - most of the time.
It can be taken with the sense of "loner, sad jonny-no-friends", as if you are implying that they are the sort of person who no-one would want to know. It seems most likely to be taken like that by people for whom this is partly true.
So take some care over how it is used. But, for the majority of people, it is not a problem.