Yeah, I would force a 50 user limit per day or whatever makes sense. The PHP component would have to know which people were already invited and/or are already users and then allow the user to select up to 50 people per day that are not on those lists. Something like that. The site administrator would control the user limit per day.

I know about the API it will still import email addresses and put inside invites field to send invitation. Besides that you can not trust user he can invite 10K users and you'll be blocked by provider, these type of things can be created but we need to take care of people with normal hosting providers they may use these blindly and ended up to account termination.

You can test the API directly here
https://developers.google.com/people/api/rest/v1/people.connections/list

For Starters, here is the API document for Google. https://developers.google.com/people
It appears it would use the user gmail address as a ID to retrieve attributes from the users contact list.

Ok, i am here telling about my contacts list a email contacts list in my mailbox (not a emails) I have 50K almost contact list.

Also, in reference to Arsalan's statement, the user would have to singularly pick up to N number of users one at a time to add to their list.

I think I misunderstand your question. I now realize you are talking about the user's email account and not those he is inviting. So, I think the answer will be a few different scenarios.

1. Access the users local email on their computer or mobile (this is probably reveal in the scripts that I will post)
2. Direct access to the users contacts by use of the email address that is bound to their account. Again, I think the scripts will reveal ways in which this is done.

Well I look at bigger picture first, my email have around 50K emails (distinct) and I wouldn't just export them because this way your hosting provider will surely suspend your account sending 50K invitations 🤣 🤣🤣🤣

Thank you for responding, by the way.

OSSN would not necessarily need to "know" if the email addresses belong to a current user, just like it does not know now. Currently, if I place [email protected] in the "Invite Friends" list, [email protected] may already exist under a current user account. So, however OSSN is handling that now seems to be fine. I will find you the php scripts and post them here.