Hi @m.siusko
According to the documentation:
In the user creation payload, user groups are only supported when importing or POSTing a single user at a time. If you attempt to create more than one user while specifiying user groups, you will not recieve an error and the users will be created but no user groups will be assigned. This is by design and is limited because of the many-to-many relationship between users and user groups whereby user groups is the owner of the relationship. To update or create mulitple users and their user groups, consider a program to POST one at a time, or POST all users followed by another action to update their user groups while specifiying the new user’s identifiers.
Therefore I refer you to the Method #2 - using Postman scripts in the guidance which I created here Bulk passwords update - #2 by Ulanbek.