In your ESTA application form, there will be a field point of the contact person. Many attendees come to the US for ETHDenver or to visit numerous attractions. If you don't have a person to write down as a point of contact, you can mark this field as Unknown. However, you should fill in the details about your first overnight stay. That can be a hotel, B&B place, or even Airbnb. You will need to fill out the correct information with the address and phone number of the hotel.
What if you don't know or haven't decided where you will stay? You can leave this fill empty or unknown also.
If you have a known person, friend, or family as a point of contact, this will increase your chance of getting ESTA approval quickly. You will have to give a point of contact home address, phone number, and valid ID number. This logic also applies in case you fill out your first overnight stay.
If you don't have a point of contact nor first overnight stay, it might make it more challenging to get ESTA travel authorization. Simultaneously, it doesn't mean you will be automatically turned down.
One field you can't leave empty during the online application process is an emergency contact. Ideally, the point of contact person could also be your emergency contact. The emergency contact doesn't have to be American, nor in the US. It can be your overseas friend, family, or co-worker.
ETHDenver cannot act as your Point of Contact for your ESTA unless we are financially responsible for your travel.
Official ESTA Website and FAQ – https://esta.cbp.dhs.gov/faq?lang=en