How to add a LDAP user to a system group (or vice-versa)

Yes they are, but there’s a common “issue” (not related to yunohost specifically) that /usr/sbin (or similar path) may not be available in the $PATH variable by default.

So maybe run sudo su to become root (though not 100% sure that prevent the path issue)

I’m not sure adduser/usermod works on LDAP entities though