Hi Alex,

I assume that you are working with the res.users list. So, one row for each user. You must have at least two columns. "id" and "groups_id/id".

Example:

"id": base.user_admin

"groups_id/id": base.group_erp_manager,account.group_account_invoice,base.group_partner_manager,base.group_user,product.group_product_variant,base.group_system,account.group_show_line_subtotals_tax_excluded,base.group_no_one,account.group_products_in_bills

Odoo expects that in column 'groups_id/id' you will add all the groups that define this user. Not just the new ones.

There's no option to just add one extra group to a user. 

I'd recommend that you use the module https://github.com/OCA/server-backend/tree/12.0/base_user_role.

This module would allow you to standarize the roles in your company, and just assign users to predefined company roles.