It is possible to activate a user, but it won't be added to Registered Users

Steps

• Fill the database with a lot of users (20K)
• Register several users
• Simultaneously activate all of them users

AR: user is activated, but it's not in Registered Users group
ER: if user is activated, it should be in Registered Users in 100% of cases

Note, that it might be hard to reproduce this issue manually, but our system tests reproduce this fairly often thus it might be better to run them couple of times in order to make sure that the bug is fixed. When this bug is reproduced, user cannot see TestBranch.