If you are in the group details, to which you have access by clicking on the group name in the
left hand menu panel.
You may now go to the settings of the group, which are located in the upper right of the screen
and click on Archive.
A confirmation dialog box will be displayed, where you can now click on the Archive button
to complete the process!
Note that an archived group is no longer accessible or changeable. However, you can
still access its conversations via the
search for example.