Can you let me know how I whitelist an email address?
That would depend on your email service. You have a custom domain so I can't even begin to guess.
If you mean at the input to your Groups.io group, there isn't a whitelist nor any need for one. Groups.io does not use content filtering, RBLs, or other techniques which might need to be bypassed. ?
My group seems to have issues with .org addresses.
What kind of issues? Trouble joining? Trouble posting? Trouble receiving group messages?