In Windows 98 the address book is a *.WAB file like IronDuke said. Usually located in something like C:\windows\application data\microsoft\address book\ and is usually named by the name of the Outlook Express user, such as "Jonny.wab" for example. Just do a seach for "*.wab" you will find it.
Note that the WAB file is completely "self opening", just double click it and it will pop up your address book. So when you find the right file, zip that up and E-mail it.
On the other end, if you don't feel like finding the WAB and replaceing it, I believe there IS an "import" function so you can just tell it to pull all the addresses out of the WAB file you copied. Have them go into the Address Book in OE and click File-import or like. Then just select the file.
On XP it is a very similar technique, only the WAB file is located in the profile directory instead of the Windows directory.
If you are talking about E-mailing an address book so that it install automatically, not so. They will have to swap files or import it.