Who joins Ralph and Jack on the 'mad expedition' up the mountain?

Roger

I believe most of the boys are there except Piggy and the "littluns".

From the text;

Side by side, watched by silent boys, the two started up the mountain. Ralph stopped. “We’re silly. Why should only two go? If we find anything, two won’t be enough.”

There came the sound of boys scuttling away. Astonishingly, a dark

figure moved against the tide.

“Roger?”

“Yes.”

“That’s three, then.”

Just before that they are on a pig hunt though. Then the boys are up the mountain and play at what they think would be a cool fort.