You can assign your contacts either one by one or en-masse.
To assign contacts one at a time, go to the Employee Contacts page and click the pencil icon next to the contact. Below your contact's personal information are the assignment and messaging preference options.
If you want to mass-assign all or many contacts, navigate to the Buttons page of your dashboard. Click on 'Visitor buttons' on the right then 'edit' for the button you want to assign contacts, scroll down to the 'Select Employee Contacts' section. Now you can select all (or check boxes for specific contacts)!