Just a rule on the field. So, I'm not sure what version of InfoPath you are using, but if 2007, you can right click on the field and select rules to look at the rules or in IP 2010, select the field and click Manage Rules from the Home tab of the ribbon.
But, really, if they add new rows after the field has been set, they won't have that value. So you may want to use a default value. Just select the field in your repeating table you want to set to the other field (field1 in this case) and right click it. Select the properties. The first tab has the default value setting - you can set it to the value of the field you wish to populate it with. See attached.