It's probably worth asking this question in an Outlook Express group as
there may be tools designed to do it, but the simplest approach I can see
that doesn't require any third-party software is:
a. Use OE File|Export to export the existing address book to a new .wab.
This should export the entire address book
b. open the new .wab (e.g. double-click it in Windows Explorer). Make
absolutely sure you are looking at your new .wab
c. delete everything except the Group you want
d. move the contacts in the Group to one of the top level folders in the
wab
e. export the wab as a text file.

An alternative would be to create a new .wab with nothing in it, then select
all the members of the grop and drag-and-drop them to the new .wab, then
export. But unfortunately if you do that, you have to accept the details for
each contact.

--
Peter Jamieson - Word MVP