How can I make a field required conditionally for a specific group of users?

Using a Data Restriction to make a field 

Conditionally Required by Security Group

• In the Security Groups application there is a Data Restriction tab.
• In this tab there are three tabs, select the Attribute tab.
• The attribute tab allows you to create an attribute level restriction.
• Select the Attribute tab and select the New Row button.
• Enter the object value for the table the attribute belongs to.
• Enter the attribute value.
• If you want the restriction to be specific to an application enter an application value.
• Leave this blank for it to restrict in all applications it appears in.
• Select the type, in this case Required
• Use the double arrows next to the Condition field to launch to the Condition Expression Manager application to create your condition or if you already have a condition associate it.
• Save
• After all active user sessions that existed before the change end, the users will see this change take effect. ( Better to re-start Server once )