This is called a conditional value list.
Define a new relationship:
Contact::Company_ID = ContactSameCompany::Company_ID where ContactSameCompany is a 2nd table occurrence of Contact.
Define your conditional value list like this.
Specify ContactSameCompany::ContactID for column 1, ContactSameCompany::ContactName for Column 2.
Specify "show only related values starting from Contact" at the bottom of this dialog.
Note: To create the second table occurrence, click Contact in Manage | Database | Relationships, then click the button with two green plus signs. Double click this new table occurrence box to change it's name from Contact 2.
That has worked a treat thanks.