On a newly installed instance, I created an empty team group with a primary option enabled. Then using a bulk editor I added a list of emails of inexistent users (no one from the list existed, 19 emails total, team members). The system reported that I need to check if users exist, and as a result, nobody added to the group.
After that, I created users (>1000) using API (I used pydiscourse module in python). 4 users from 1000 (incliding 19 members) created as inactive and the system sent them activation emails. I find such behavior weird.
If I first create users and then add 19 members to a group, everything is ok, everybody is active.