I'm assuming you have looked at the help docs??
Invite and add members—ArcGIS Online Help | ArcGIS
there are some tips in there, including adding them without sending the email, etc.
But have you looked at the new Python API? That is another way to add users and do other maintenance. It takes a little time to see the pattern, but there are samples and it's actually pretty powerful once you get the hang of it (and there are samples).
ArcGIS API for Python | ArcGIS for Developers
https://blogs.esri.com/esri/arcgis/2016/12/19/arcgis-python-api-1-0-released/
https://community.esri.com/groups/arcgis-python-api/pages/overview
With the API, you can even create a new user or group, assign level, and move content from one user to another. This is what I am using it for, to move content from a user htat has left the Department, and assign their content to a holding user until it can be sorted out, then deleting the old user.
There some good sessions showing this process at the Dev Summit 2017 (last month). It should be posted on video.esri.com but I'll see if I can find a direct link.
There may be other admin tools that can help too...
Resources · Esri/ago-admin-wiki Wiki · GitHub
Tools · Esri/ago-admin-wiki Wiki · GitHub
GeoJobe has some free (or more advanced, paid version) that many users seem to like. I have played with the free version.
Keep in mind that you will want to make sure you have enough named users for all those you are adding, and will want to designate whether they are level 1 or 2, etc.
edit:
Recent Proceedings (for the Dev Summit)
Esri Events - YouTube (if you prefer youtube)
Esri Videos | E360 Videos: GIS, Events, ArcGIS Products & Industries (the official esri video site...you can search for topics).
... filtered for python api E360 Video Search | Esri Videos