It seems that an admin can invite a user to a group just using the email address from the gitlab ui, but there is no way to currently do that using the gitlab rest api. I’m looking at the documentation here https://docs.gitlab.com/ee/api/members.html#add-a-member-to-a-group-or-project, currently we can only invite existing gitlab users and we need to know their id to be able to invite them.
It would be nice if the invite by email functionality was available on the REST API as well.
Hey @niinpatel, welcome to the GitLab Community Forum! 
You’re correct that it’s not yet possible to invite a user via email to a group or project via the API just yet. However, we do have an open feature proposal issue suggesting that an endpoint be implemented for that.
I’d encourage you to give that issue a thumbs up, which will allow you to receive any updates on it and feel free to leave any thoughts you have about that proposal in the comments section there.
Thanks for the reply, but it looks like the ticket you linked is open for 3 years and its unlikely that anyone’s going to take this up in near future 