When managing users with Ansible's user module, properly specifying group memberships is crucial. The error message about "key=value arguments" typically occurs when the YAML syntax for the groups parameter is incorrect.

Here's the proper way to specify multiple groups in both YAML formats:

# Correct multi-line YAML syntax
- name: Create user with multiple group memberships
  user:
    name: testuser
    state: present
    groups: "group1,group2,group3"
    append: yes
    comment: "Test user account"

# Correct inline syntax
- name: Create user with multiple groups (inline)
  user: name=testuser state=present groups="group1,group2,group3" append=yes comment="Test user"

The groups parameter expects a comma-separated string, not a YAML list. Common mistakes include:

# WRONG: Using YAML list syntax
groups: ["group1", "group2", "group3"]

# WRONG: Using multiple quoted strings
groups: "group1", "group2", "group3"

# CORRECT: Single string with commas
groups: "group1,group2,group3"

Use the append parameter to control whether the user remains in existing groups:

- name: Add user to additional groups without removing current memberships
  user:
    name: testuser
    groups: "newgroup1,newgroup2"
    append: yes

- name: Replace all group memberships (only keeping specified groups)
  user:
    name: testuser
    groups: "onlygroup1,onlygroup2"
    append: no

Here's a complete playbook example for adding a user to multiple system groups:

- hosts: all
  become: yes
  tasks:
    - name: Ensure webadmin user exists with proper group memberships
      user:
        name: webadmin
        shell: /bin/bash
        groups: "www-data,adm,sudo"
        append: yes
        comment: "Web Server Administrator"
        createhome: yes
        system: no

After running your playbook, verify the group memberships:

- name: Verify group memberships
  command: id webadmin
  register: user_info
  changed_when: false

- debug:
    var: user_info.stdout

For more complex scenarios, you can use variables or conditionals:

- hosts: all
  vars:
    user_groups: "{% if inventory_hostname in groups['web_servers'] %}www-data,nginx{% else %}staff{% endif %}"
  tasks:
    - name: Configure user with dynamic groups
      user:
        name: deployer
        groups: "{{ user_groups }}"
        append: yes

When working with Ansible's user module, many administrators stumble when trying to add users to multiple secondary groups. The error message "this module requires key=value arguments" typically occurs due to incorrect YAML syntax rather than a functional limitation.

The proper way to specify multiple groups is as a comma-separated string without additional quotes around individual groups:

- name: Create user with multiple group memberships
  ansible.builtin.user:
    name: testuser
    groups: "group1,group2,group3"
    append: yes
    state: present

groups: Accepts a comma-delimited string (not a list) of supplementary groups
append: When yes (default), preserves existing group memberships when adding new ones

For dynamic group assignments from variables:

- name: Add user to groups from variable
  ansible.builtin.user:
    name: "{{ username }}"
    groups: "{{ user_groups | join(',') }}"

When you need to replace all existing group memberships:

- name: Set exact group membership (overwrite)
  ansible.builtin.user:
    name: service_account
    groups: "adm,wheel,www-data"
    append: no

If encountering persistent errors:
1. Verify YAML indentation (2 spaces recommended)
2. Check for hidden special characters in group names
3. Use ansible-playbook --syntax-check to validate playbooks
4. Test with simplest possible playbook to isolate the issue

This complete playbook demonstrates proper multi-group assignment:

- hosts: all
  become: yes
  tasks:
    - name: Configure web admin user
      ansible.builtin.user:
        name: webadmin
        comment: "Web Administrator"
        groups: "sudo,www-data,backup"
        shell: /bin/bash
        create_home: yes

Remember that group names must exist prior to user assignment. Consider adding a preceding task to ensure group existence if needed.