With the release of DarkRadiant 2.2.1 the grouping feature was introduced. This feature lets you group together several individual entities, so they are treated as a single entity in DarkRadiant. In order to group entities, select all entities, that you want in one group, and then either use the right mouse button and select "Group Selection" or use Edit -> Group Selection in the task bar. The group can be disbandend by using "Ungroup Selection" in the same way. Note that it is not possible to change individual parts of the group without ungrouping.
Grouping and Prefabs
With the introduction of the grouping feature Prefabs are grouped by default, as they ususally are used as a single entity. There a re examples, where this behaviour is unwanted (e.g. for the key of a chest prefab). It is possible to change this behaviour before inserting the prefab by unticking the Box "Create Group out of Prefab parts" in the "Choose Prefab" window. Please note, that this box remains unticked after inserting the prefab; so if you want to insert a prefab as a group later on, you will have to tick this box again.