Though going through the 'friends method' explained by Shrude is best for frequent mutual games, it's not necessary.
In the initial game screen click on INVITE PLAYERS, then click CREATE GAME. In the new game setup window that appears, click on Manual in the first player selection box. It'll look like this:
Type in the first few letters of the playername, then click on the search box. If your dad is logged in to WGT, AND available to play, he'll receive an invite. If those same initial characters are shared by more than one WGTer, then he'll appear on a list of possible players that pops up.