So you have few people who are not linked automatically to an Account. You want to manually link to an account. They would have not been linked to an account because their email id is of Gmail or Yahoo or etc. You do not have official email id. In order to link a contact to an Account do the following:

Click on the Account to view the Account Profile Page. 

Step 2: In the Account Profile Page click on Contacts. 

Step 3:  Click on Link Contact as shown below

Step 4: Now the Window will open to Link contact with an Account.