usermod returns "group 'thegroupname' does not exist" for known group

Question

As a member of sudo can I add myself to a group I don't belong to?

$ getent group thegroupname 
thegroupname:x:123794798:administrator,users,auser,moreusers,manymoreusers

$ sudo usermod -a -G thegroupname myusername
usermod: group 'thegroupname' does not exist

When I run the above I can't add myself to the group. The error message says the group doesn't exist, but it does.

Here I'll make a new group as per @terdon's comment.

$ sudo groupadd testgroup
useradd: user 'myusername' already exists
$ groups
myusername sudo users

$ getent group testgroup
testgroup:x:01234:


$ sudo usermod -a -G testgroup myusername
$ groups myusername
myusername : myusername sudo users testgroup

Sorry for the typo with just using groups instead of groups myusername. I'm not sure if that information is helpful as it still shows me as not belonging to the test group while groups myusername does.

$ groups
myusername sudo users

Solution

I was trying to add a local user to an Active Directory group. This is not allowed.

Comments

[terdon]

I suppose that creating a new group and adding your user to it does work as expected right? It's only thisgroupname that fails?

[Louis Waweru]

@terdon I'm not sure. I could use usermod but not groupadd. I've updated the question based on your feedback.

[Louis Waweru]

@terdon reading that again, it seems the answer to your question is "yes".

[terdon]

Hmm. Strange. Could you fix the typo you where groupadd prints a useradd error message (2nd code line in your edit)?

[Louis Waweru]

@terdon Doh, I overlooked something obvious trying to add a local account to an Active Directory group. If you want to post that in an answer, feel free! Thanks for your troubleshooting.

[terdon]

Huh? You mean the issue was thta you were running sudo useradd testgroup instead of groupadd? If so, lol, and nah, not worth posting an answer, just delete it. It's unlikely to help anyone else.