I assume by "group" you mean a folder under Contacts.
In Outlook 2000

To Export into Excel:
File Menu
Import and Export
Export To A File (Next)
Microsoft Excel (Next)
Select the folder, AKA group, to export from (Next)
Give it a file name. (Next)
(Optional but just to play it safe)
Map Custom Fields button
Default Map Button
OK
Finish

To Import from Excel
File Menu
Import and Export
Import from another program of file (Next)
Microsoft Excel (Next)
Give it the file name. (Next)
Select the folder, AKA group, to import it to (Next)
(Optional but just to play it safe)
Map Custom Fields button
Default Map Button
OK
Finish