Expose the Contacts tab on Lead Records

To expose the Contacts tab on a Lead Form, perform the following steps:

1. Navigate to Customization > Forms > Entry Forms.
2. Customize the Standard Lead Form.
3. Specify a Name for the custom form.
4. Check the following boxes: 
    a. Form is Preferred
    b. Store Form with Record
    c. Enable Field Editing on Lists
5. Click the Lists tab >Relationships subtab
6. Check the Show box for Contacts
7. (Optional) On the Roles tab, set the Form as Preferred for any custom or standard role.
8. Click Save

The ability to add/view Contacts can be done in Relationships > Contacts tab on the Lead record.