Hi,
I added some users to my organization and it worked well as usual with one exception: I get the message “Sorry, <email> cannot be invited <organization>.” and I don’t know why.
Adding via username is also not possible; the username is shown greyed out and I cannot select it.
Has anybody an idea why this can happen and what I can do to find out the reason?
Thanks in advance.
@t7l Because figuring this out might involve talking about specific account names and taking a look at your organization account, I think we’d be better able to help via private email support. Can you send us a message at git.io/c so we can take a closer look? Thanks!
Sure, thanks for the reply. 
Hi Nadia,
Any info you can share as to the resolution? This is happening to us as well.
Dan
Hi @danchisholm,
As mentioned above, because figuring this out might involve talking about specific account names and taking a look at your organization account, I think we’d be better able to help via private email support. Can you send us a message at https://github.com/contact so we can take a closer look? Thanks!
Same here. It would be good to have a troubleshooting on this.
The user seems valid, does not have a pending invite and is not already in the org.
I had the same problem as you. I clicked “Opt out” button, that’s why I was not be able to get invitation. It was resolved by writing to the support: https://support.github.com/contact. They removed my opt out and I have got invitation.
I’m having the same issue this week. I’ve created a support ticket, but a user interface option to opt back in would have been helpful and would probably save GitHub some money by taking another task off support’s hands.
.