To place elements in a group, in Project Explorer right-click the elements to place in a group and select Move to Group... from the context menu. This will bring up a small dialog allowing you to specify the name of an existing or new group.
You can also change the name of the group an element is in, in the specific editor for the element, e.g. the Entity Editor. When no elements are present in a given group, it's automatically removed.
To rename a group, in Project Explorer select the
group node and press
F2 or right-click the group node and select
Rename... from the context menu.