Your files are not in YAML- Syntax. Ansible should give you an error if you start the playbook. You dont event need it at all.
Use with_lines! Like this:
tasks:
- name: Create groups
group:
name: "{{ item }}"
state: present
with_lines: cat groups.yml
- name: Create users
user:
name: "{{ item }}"
state: present
with_lines: cat users.yml