Nancy, Thanks for the reply. I think "Team Site Members" and "Team Site Vistiors" are groups in the E1 or E3 plans. I am in a trial of the P1 plan and they are listed as just "Visitors".
The suggestion though would be to have the invite dialog configurable so that I can choose the group I want to assign the external users to. That way they only have access to the sub-stie as soon as they accept the invite.
Thanks,
Irvin