register all data from group into a single variable

- hosts: localhost
  connection: local
  tasks:
    - name: setup parallel workers
      add_host:
        name: task-{{ item }}
        groups: parallel
        service: "{{ item }}"
      always_run: yes
      with_items:
        - one
        - two
        - three

- hosts: parallel
  connection: local
  tasks:
    - name: build data
      set_fact: var={{ hostvars[item].service }}
      delegate_to: localhost
      run_once: yes
      register: services
      with_items: groups['parallel']
    - debug: var=services
      delegate_to: localhost
      run_once: yes

last task produces:

ok: [task-one -> localhost] => {
    "services": {
        "changed": false, 
        "msg": "All items completed", 
        "results": [
            {
                "ansible_facts": {
                    "var": "one"
                }, 
                "invocation": {
                    "module_args": "var=one", 
                    "module_name": "set_fact"
                }, 
                "item": "task-one"
            }, 
            {
                "ansible_facts": {
                    "var": "two"
                }, 
                "invocation": {
                    "module_args": "var=two", 
                    "module_name": "set_fact"
                }, 
                "item": "task-two"
            }, 
            {
                "ansible_facts": {
                    "var": "three"
                }, 
                "invocation": {
                    "module_args": "var=three", 
                    "module_name": "set_fact"
                }, 
                "item": "task-three"
            }
        ]
    }
}

Which I can then use map on to extract ansible_facts.var which is the piece I'm interested in.