How to add a local group?

Why a local user group cannot be added on ESXi 5.1?

I can add local users, but not groups.

But adding a group fails with error: "The operation is not supported on the object".

Call "HostLocalAccountManager.CreateGroup" for object "ha-localacctmgr" on ESXi <ip addr> failed.

I have the latest ESXi 5.1 on a standalone Dell PowerEdge server and latest vSphere client build 1064113 (in 60 days evaluation mode).

I've tried to specify group id (like, 2000 or 3000) or leave the field blank, specify existing user to add or leave blank - no effect.

Thanks,

Pavel A.