Yes.  This can be done.

Create a profile field called newsLetter group and make it a yesNo field.

Next, make your newsLetter group, and in the SQL Query field, put:

select userId from userProfileData where newsLetter=1

Now, the members of the group are determined by their profileField.  You won't be able to list the members of this group through the group interface, but they'll be in the group nonetheless.

If you want to make a list of members, use the SQL Report.

--- (Edited on 9/7/2008 8:32 am [GMT-0700] by perlDreamer) ---

With   you can even add spaces to the profile field.
e.g.:
as the label of the profile field put:

• LoopSetShop Newsletter

and with SQL query in the edit group page put:

• select userId from userProfileData where LoopSetShop Newsletter=1

Since   will appear as a space in html.

--- (Edited on 7-September-2008 18:13 [GMT+0200] by LoopSetShop) ---