Hello Shaun,
If you are using CCA, it should automatically subscribe the users to the GDM. If for some reason it did not, I would try removing the blast group and then recreating it.
If you are NOT using CCA, I would go into the CUE web interface (10.1.10.1 in web browswer) then modify the users, and add them/subscribe to the blast groups GDM.
Thank you,
Darren