Co-Authors Plus issue

Hi guys,
I need to assign more than one author to the same post, and I’ve found out a couple of ideas:

– using ‘Author’ category with authors as subcategories: it’s fine, but then i have to render ‘normal’ categories and ‘author’ categories in separate ways. It’s probably feasible but it looks messy.
– Co-Authors Plus plugin: it looks perfect but when I try to activate it I get this error:

Fatal error: Call to undefined function wp_set_post_terms() in /web/htdocs/www.ilcontesto.org/home/wp-content/plugins/co-authors-plus/co-authors.php on line 282

Plugin site: http://ww.wp.xz.cn/extend/plugins/co-authors-plus/

What am I supposed to do to fix this issue?

My wordpress version: 2.7.1.

Thanks in advance!
Mulone