The North Peak of Guye is the highest point on the mountain and the true summit. For some reason, the register is located on the South Peak, and Fred Beckey's "Cascade Alpine Guide" says, cryptically, that the "S summit is the usual destination". However, any sight level (or looking through a water bottle) will show that the middle and south peaks are both a few feet lower than the north peak.
The spot elevation on the topo map is 5168 at the South Peak, so the North Peak is about 5170 feet high.
The unmaintained hiking trail up the north ridge from the Alpental parking lot leads right to the north peak, so all hikers that tag the highest rock point at the end of the path should be able to claim full credit for an ascent of Guye peak.