Most citation/reference management applications provide options for sharing references to promote collaborative writing. This is becoming an increasingly important feature of new releases.
EndNote Basic is a free light weight online reference manager that can be used on its own, or in conjunction with EndNote Desktop. Basic allows you to share your library with colleagues, including notes and annotations.
Zotero has a robust sharing feature. It takes more set up than others.
The basic steps. 1. In Zotero online set up a group, 2. invite others to the group. 3. In Zotero desktop add the articles you want to share to a folder. For more details see Zotero handout.
Sciwheel allows robust sharing. You can share with others who have a Sciwheel account. See image to right.