Overview: By default, contacts and companies have no owner and are therefore visible to all users. Assigning them owners, however, will allow certain contacts/companies to be viewed only by those assigned. The process is the same for both contacts and companies, this How To will use a contact as an example.
Steps:
1. Click on the ‘Contacts’ Module
2. Click on the ‘contact’ that you wish to assign.
3. Click on the ‘edit pencil’ from the top menu.
4. Scroll down to the More section, select the ‘Owner’ field and choose which users(s) you wish to assign. 
5. Once users have been selected, the user will be shown on the contact's info and any standard users that aren't Owners will no longer be able to see the contact.
6. An alternative method is to use the Assign to button in the top menu, simply select one or multiple contacts/companies from the list.
