I might just know the reason but i am confirming. like subscriptions, do reminders allow using a user field? For example i am using [PM] but they are not receiving this.
For anyone who finds this, groups appear in the Browse Users pop-up window. Select the group and the Users in the group appear in the left-hand select list. Select one at a time and click Add. Then click Done. The selected Users email addresses are added to the "Specific List of Users" Text field.
It is not dynamic, which is what one would expect from the concept of Groups in other software Applications.