they will line up whenever you issue a move command into "nonhostile" territory. Basically, if they have been issued a move command and haven't engaged they will line up all pretty.
Regroup simply means that the angels become "reattached" to you and will copy whatever you do. Want to kill some creeps? They'll be happy to oblige.
What you want to do is push "Select Special" (don't know what that is in the default layout, I have it remapped to
then click somewhere behind you. Of course, don't forget to reclick Oak so you issue him order and not your spirits.