Run task only if host does not belong to a group

Here’s another way to do this:

- name: my command
  command: echo stuff
  when: "'groupname' not in group_names"

group_names is a magic variable as documented here: https://docs.ansible.com/ansible/latest/reference_appendices/special_variables.html :

group_names is a list (array) of all the groups the current host is in.