Email to parent group doesn't address sub-groups

While I can see the configured sub-groups and their members, it seems impossible to send an email to an OG that consists of purely sub-groups (i.e. it shows 0 actualt memberS). However with the latest OG verison (7.1) I can see all members, including the subgroup members (i.e. it traverses the sub-groups and extracts all members), though I cannot email them.
Any suggestions, configuration settings (short of having all members to also subscribe to the parent group, which defeats the whole purpose of a subgroups)?

Thx, RK