This is something you are going to want to do and it's simple.

Adding people to your network allows you to keep track of people you are interested
in within the community. It acts as a type of bookmarking function.

Through the Network tab in your profile, you'll have quick access to members that you have added to your Network.

By adding people to your network, you can then access other functions within the
community, such as the Inbox Messaging feature.

When composing a new message, the Members of your network appear in a list for you to choose who you want to send the message to. 

If you want to send a direct mesage to a member of the community, you must first add them to your network.

To add members to your network, look for the little person icon alongside a member's name.  Simply click that icon to automatically add that person to your network.   Or, go to a member's profile and click on "Add member to my network."

They will receive an email notifying you have done so.