group_vars--all.yml

bar: 'bar'
foo_json: '{{ foo | to_json }}'

group_vars--dummy.yml

foo: 'foo'
bar_json: '{{ bar | to_json }}'

hosts.yml

[dummy]
127.0.0.1

playbook.yml

- hosts: all
  tasks:
  - template: src=template.j2 dest=/tmp/template

run.sh

ansible-playbook -i hosts.yml playbook.yml

template.j2

{{ foo_json }}
{{ bar_json }}