Just came across this, and I agree that signup rates would likely improve if people were able to sign up with a social account after receiving an email invitation. Many ‘occasional’ users are reluctant to sign up with email and password because it’s another password they have to remember.
We typically invite people to specific groups, so I can see the challenge in inviting them using email, and they then sign up using a social account that may not have the same email address etc. At the same time, it would certainly remove a lot of friction in getting occasional users to sign up.