I don't know if this is common knowledge and I was one of the few ignorant ones, but Groups.i.o has a top level Account Identity and an Identity specific to each group you join. The default setting is for no-one to see your email address, and that means you can only be contacted through a board posting. Mine was set to Only Moderators and Owners and I didn't realize it. You access all your profiles stating by clicking your name in the upper right.
By selecting the [Identity] button on the left you will see your top level profile and then a profile option for each group you are a member of. When you select [Edit] for your top level profile, there is a note at the top of the page:
"Changes made to this profile will also be made to each group profile, with the exception of those fields that you have previously customized in each group profile. To edit those customized fields please go to the specific group profile and click "edit" "
About 1/3 down the page is [Profile Privacy]. The profile email settings start out set to Moderators and Owners only. If that setting is never changed, it's adopted into each group you join. If you change it, then it will change settings as stated above.