[Typo3] Email notification

[Malcolm Wearing]

I am trying to piece together how to create this scenario in Typo3 and would
appreciate some input:

Two types of usergroups, UserGroup A and UserGroup B.

UserGroup A is free to register but UserGroup B pays to advertise their
services.

When UserGroup B registers with a particular service or skill, a
notification email is sent to all of the members of UserGroup A who have
already requested to be notified about this. UserGroup A can also choose not
to be notified during registration if they want.

I have been scouring the List and all the available extensions trying to
find a clue about how to go about it - any ideas?

Thanks
Ed