Referencing current variable in Ansible conditional statement?
How about something like this:

hosts: all

vars:
  user_servers:
    user1:
      - testing1

  users:
    -name: user1
    -groups: '{{ user_groups }}'
    -state: present

roles:
  - name: Add filtered users
    tasks:
      - name: Filter users 
        debug: User {{ item }}
        with_items: "{{ users | default([]) }}"
        when: {{ inventory_hostname_short }} in user_servers[item]
        register: filtered_users  

      - name: Add filtered users
        include_role: wilshersystems.users
        with_items:
        - {{ filtered_users }}
        loop_control:
        - loop_var: users