There are two ways to resize a Group
Shift and drag
Hold down the SHIFT* key and click-and-drag the border of the Group to resize. This not an option on touch devices.
Square borders have on the top-left and bottom-right corners a resize handle (grey circle) that can be click-and-dragged or touch-and-dragged to resize or reshape the group.
Click-and-drag or touch-and-drag the border path of a group to move it around the board.
Tip. Only the border path will move the group. Clicking/touching the Group Title or the contents within a Group will only move those items.
Moving just the Group and not its contents
By default when you move a Group all of the contents (including items that just touch the border) will move with the group.
However, if you want to just move the Group (the border and title only) then first double-click or double-tap the Group border or title to bring up the Edit Panel.
In the Edit Panel click/tap the Items tab at the top. Here you will see a check-box next to the Stick to this group, uncheck this to move the Group without moving the contents.
You will notice that the border now has a dashed line to indicate that it's contents will not stick to the group.
Tip. You can create a temporary group around multiple items to move or delete them rather than moving or deleting them one at a time.
* Rather than using the SHIFT key you can also use the ALT, CTRL or CMD keys.