To do this task you will need sufficient permission in the site and library. Go to the Document Library and click on the Settings button > Select the Document Library Settings
You should now be in the Document Library Settings page. You can customize the Library in this page. Look for Incoming e-mail setting link under the Communications tab.
*Note: If you are not able to see the Incoming e-mail settings link it means that you do not have you incoming email settings configured in the server. Contact the admin for this or visit this link if you wish to do it yourself http://blogs.msdn.com/selvagan/archive/2008/01/26/incoming-email-configuration-moss.aspx
Open the incoming email settings link.
Select "Yes" in the "Allow this document library to receive e-mail?" section
Specify an email address for users to email the documents. Select the preference for Email Attachments. There is also option for saving the original email message and meeting requests. If you find this unnecessary skip it or leave the selection as "No".
One of the very good feature is that you need not give permission to users to send the documents/attachments as there is an option that allow users to send the document without having permission in the document library.
Once done click OK. It's good to go now. Try sending a mail with attachment to test it out.