You will notice in Outlook that you can export the current address book
to a text file. If you analyse this file, you will notice that it is in
a simple database format.

Once you have done this, you can then experiment with importing data
from an Excel file  into Outlook. Setting up your mailing group after
that should be quite easy.

Just before you import, though, perhaps you should consider in what
format your Excel data should be in to make it Outlook Import friendly.
Per haps, First name, Surname, Email Address would be an appropriate
structure.

Also, on one of the import screens is a button labeled "Map Custom Fields"
that will allow you to specify what Excel column name goes with which
Outlook contact field. Outlook takes an initial guess and if you used the
same names created when exporting the Outlook addresses, there should be
nothing to do unless there is a special Excel column.