How do I rearrange groups in the webdock?

You can drag groups up or down to reorganize them. When you start dragging a group icon, a dashed line will appear, showing you where to drop it.
You can merge groups by dragging and then dropping over another group. A pop-up will appear to check that you want to merge them.
If you use dividers, note that moving a divider doesn't move the groups that collapse into it. There's a shot KB on how to rearrange groups with dividers: https://wavebox.io/kb/how-do-i-use-dividers-in-the-webdock/

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.