The networking part of LinkedIn is useful for recruiting or prepping for a sales call or for locating the right person within a company or community. You can access other people's information so long as you are connected to them in some way. The number of connections you have is powerful. You use the Add To My Network functoin to connect to someone you know.
You'll want to seed your connection list by uploading the address book from your email program. LinkedIn will look for other LinkedIn members with those addresses and start the process of connecting them to your new account. For those email addresses you have which LinkedIn does not know yet, you can have LinkedIn send each person an invitation to join.
Once the other party has confirmed you as a connection, you should view their list of connections and use the "Add To My Network" feature for everyone you recognize.