There is support in groups.io for users to self-service and merge two ids and preserve all subscriptions and messages under one email id.   

• You sign in to groups.io using the email id you don't want anymore.
• You go to account (Top Right of screen). In there you change the email address to be identical to the one you do want and hit Save.
• A special prompt pops up asking if you want to "Merge" the two Ids.
• If you say yes, the "good" account will be sent an email with a link in it.
• You have to click on the link, sign in as the desired userid and say "Yes" to Merge in the old id. 

Your two identities will be merged with all the content and subscriptions from both under the good email address and the id you didn't want will be deleted. 

Adam