To share a project, go to Manage View, mouse over the project you want to share, click on the pencil icon to pop open the project properties window and add email accounts to the list. For each email you can add Wireframe editing rights, Manage rights or only Preview if nothing chosen. An email is sent to each account added to notify them. Wireframe editing can only be done by paid members.
To access a shared project, go to Manage screen, click on the second tab called Shared, and you will see the shared projects listed.
Alternatively, if you don't want someone to login to view just choose "Generate Preview URL" (Share), which is the 5th icon from left in wireframe mode, and send someone a URL to view it. They won't be able to comment on it, just preview and click any links you have enabled. Additionaly you can view this video tutorial Sharing wireframe
to see how to do this.