The Fillet Pipes command is used to join two pipes together. This command works similar to the standard CAD FILLET command.
To fillet two pipes, go to
Ribbon: DM Plumbing->Pipes-> 
Fillet Pipes
Pulldown Menu: DM Plumbing->Pipes->Fillet Pipes
You will be prompted to select the first pipe to fillet.
Select first pipe:
The selected pipe will be highlighted. You will then be prompted to select the second pipe.
Select second pipe:
The two pipes will be extended to each other and connected. If the pipes are at different elevations, a vertical pipe will be be inserted to complete the connection.