Level of access

Is it possible by means of CB to add the level of access to a million registered users? On 5 thousands of this difficult.

You can use the Batch Tools in CB > User Management to add usergroups to users, but CBs and Joomlas batch tool for doing this is not likely going to handle a million users at once. You'd have to do it in several batches. Your alternative is to add them directly in the database using Joomlas _user_usergroup_map table.

Through the database will be correct. But I think I figured out how to do it through auto action. Create an additional change field, it will be set to "empty"for all users. The change value will be checked during authorization. If empty, new access levels are added and the value 1 is written to the change field. On subsequent authorization, if the change field is 1, no changes are made to the access levels. It turns out it is necessary to do two auto action?

Yes, you'll need to two auto actions for that. 1 to check the field and adjust access and another to set the value of the field.