UNION Command:
AutoCAD
Command List
The UNION command allows you to join solids to form a more complex
model.
To access the Union command you can:
type union or uni at a Command:
prompt,
OR
select the Union button 
on the MODIFY II Toolbar (IF DISPLAYED),
OR
select the Boolean, and then Union, under the Modify Menu.
Prompt: Initailizing...
Prompt:
Initializing Advanded Modeling Extension
NOTE:
Any time you call up a modeling command for the first time, AutoCAD will
initialize the modeling subprogram.
AutoCAD
will now ask you to select the items you wish to join (union). Click
on the items to select them.
Press Enter.
Return
to the AutoCAD Command List.