If your needing to include the link in emails this is fairly easy;
using the 'share' option on the button ribbon just below the project name (in the project page) you can select 'link' which will provide you with the URL for the project. Don't forget though that anyone trying to open the link will need to be an active user of your installation.
If you need to use with exterior people, then I'd suggest using the widget (Same menu option) which will give you a link that provides a 'read only' version of the project.
Hope that helps!