The import process will create all non-existing groups. If the user already exists and the "add_existing_users_to_groups" switch is NOT set, then the line is skipped (so if you specify a nonexistent group for this user, the group won't be created). If you enable "add_existing_users_to_groups" and the user is already present, the non-existing groups are created and the user is assigned to all the specified groups.
If you specify a group name which starts with a "-" (dash), then the user will be removed from that group. This feature is working even if you don't enable "add_existing_users_to_groups".
To remove users you can use the same .CSV file you used for creation, or you can provide a file with a users list (one user per line). This file will be parsed for user deletion if you specify the "--remove_users" parameter.