So far I plan to have only three admin buttons on my CP. Those being "Exit," "P1 Coin" and "P2 Coin." I notice some people mentioning P1 & P2 Start. But the start buttons are not admin buttons
Anyways here is a mock up of my layout...

I still have to redesign it so the exit button is not so close to other buttons. I plan to use P1 Start as the ENTER button.
As for accessing the admin options, I will add a hidden button underneath the CP but above the coin slots. It will be upside down. It will be a shift button. You will have to hold it down and press buttons from P1 or P2 to be able to do the admin stuff like accessing the menu, resetting the game, accessing the games service, pausing, accessing mames volume control and all that other sordid stuff. But I plan to add a switch inside the CP that allows me to turn off that underneath shift admin button.
As for the whole pausing thing, I don't believe there should be a pause option. The original arcades didn't have a pause button. I'll know how to pause it though and I'll let only certain people know how to pause it as well.
As someone once mentioned, adding the pause button can lead to trouble since a person can pause the game, walk away and render the arcade useless since other players will not want to disturb their game. Or will disturb it by unpausing or changing games, causing the person who paused the game in the first place to return and be mad that his game was turned off. So no pause button for me!