The group expansion only happens for groups in your personal NAB. You can do the following to disable this expansion when sending mail:
Modify your memo form to include a hidden field named "ExpandPersonalGroups" with Text type and Computed when Composed with a value of "0". This will cause personal groups to NOT be expanded. Changing the value to "1" will cause the groups to expand.
The other solution is to use group alias names, but this technique only works in R3.x. For example, if you have a group named Managers make it Mgrs;Managers instead. Aliases are not expanded. That way, if you want it to NOT expand send the memo to Managers. If you DO want it to expand them, send it to Mgrs.