Prefabs

If you want to save a collection of entities as a group, you just select them all, go to file-save selected as... save them as a .map, ideally called something appropriate like prefab_myobject.map

This means I can set up something like a streetlight mesh, two particle effects for the electric, a sound speaker, and the light entity for it, and then save it as one prefab object, so mappers can just drop it into their map.

To open the prefab you use file--load rather than file--open.

Notice there's also a 'save seleciton as prefab' and 'load prefab' buttons in the edit menu.

Rotating Prefabs

Hit ctrl-R to change rotation modes. They'll collectively use a single axis.