To travel to Guam requires full US entry documentation (passport & visa or visa waiver). US Citizens are best with passports, though I know some who still travel on their birth certificates.
The funny part is that if you are travelling (back) from Guam to Hawaii you clear US entry immigration while exiting Guam, before you get on the UA airplane (@5:30am).
Also, all transit passengers at Guam (even say on a [Tokyo] NRT-GUM-ROR [Palau]) must go through US immigration while transiting GUM - and they need a US transit or entry visa if they are not from a visa waiver country.
If travelling from Saipan (CNMI = US Territory with US Immigration) to Guam, you clear US immigration in Saipan before you get on the plane there also. Then GUM customs, or onto your next flight ... (If that is HNL then more immigration )