It used to be a big complement to call a woman "handsome", and I'm basing that on books and movies that were written over 100 years ago. But the usage has changed, and now "handsome" is usually used for men and "beautiful" is usually used for women. Technically you can interchange these, but it's not advisable.

Handsome means well-built and beautiful, but in a more athletic than traditionally feminine (i.e. slender and delicate) way.

@Bell - <em>elegant</em> is probably what you're looking for. Beautiful, pretty, gorgeous, etc are also fine.

"Handsome" can be used to say that a woman is attractive, but not in a typically feminine way (as Sonja has pointed out). So, a woman who is attractive and has masculine traits. I'm not sure if your friends are using the word incorrectly or not, as I'd need to see who they're talking about! Still, it is unusual for someone to often describe women as "handsome".

guapo = handsome