A common way of sharing items of interest is email. There are much easier ways to share and these tools allow new ways to share what you're reading.
How can I organize my papers? CiteULike has a flexible filing system based on tags. You can choose whichever tags you want, and apply as many as you like to a paper. You can use tags to group papers together.
Can I import my existing references into CiteULike? Yes. Create a BibTeX file containing your references and then import it into CiteULike by using the "Import" link at the top of your "Library" tab.
What are "groups"? Groups are collections of users creating shared libraries of links. They are useful for keeping track of a particular topic or what everyone else in a lab, class or academic department is reading. You can start your own groups and join existing groups.