In round 4-2, shoot at the first conveyer belt which will make a two-up box appear. Then kill yourself and repeat for as many men as you want. The game will keep track of extra men beyond what the counter shows.
At the "Options" screen set Shurikens to "00". Wait for a short time and the "00" will change to an infinity symbol.
Road Avengers (SegaCD)
At the "Start" screen press U to highlight "Options" and press start. Highlight the "players" option and press Ax6, and you will hear a beep. When you start the game you will be asked to select a level.