There are several popular search results for this answer but they all skip the important part – and are incorrect as a result.
--- - hosts: all sudo: yes tasks: - user: name=myuser comment="My User" groups=wheel,group1,group2 append=yes
This is where they are (and likely you are) going wrong. The “groups” option and the “append=yes” option. “groups” has an “s” in it. If you’re having trouble adding a new or existing user to multiple groups then this is likely the solution.