You can't.

Do you need to?  Just group by Category and to email that whole group, drag
it to the Outbox, to invite them to a meeting, drag it to the Calendar, to
email merge to them, highlight them and select tools | Mailmerge | to email.
There's a lot more functionality in grouping by Category than in
Distribution Lists.  Check out some of the older postings too.

Judy Gleeson  - MVP Outlook
Acorn Training and Consulting
Canberra, Australia

see what Outlook training can do to improve productivity:
www.acorntraining.com.au/pdfdocs/ProductivITwithOutlook.pps

www.acorntraining.com.au/productivit.htm