Right-click on a tab in the tabstrip and select 'Move to' and choose a group from the list.
You can also drag a tab from the tabstrip into a new group in the webdock.
Or, click on the webdock icon to the left of the address bar (it looks like a list) to switch to Explorer view. In this view, tabs are shown beneath groups and apps in a nested list. From there, you can drag to rearrange them and also reorganize them using folders.