If you want to add users into multiple groups simultaneously, it's just a matter of adding the groups into the Groups column of the CSV spreadsheet and separating the names with semicolons (;).
For example, if I wanted to enroll a user into Group 1, Group 2, and Group 3 at the same time, I would enter Group 1; Group 2; Group 3 in the Groups column.
Once done, add the users like you would normally with the Bulk Upload function.
To read the article about adding users in bulk, please see this article.