What forms do I need to complete at the immigration office to stay with my girlfriend in Thailand, and does she need to accompany me?

To comply with Thai immigration regulations when staying with your girlfriend, who is the property owner, you need to complete the TM30 registration within 24 hours of your arrival. While it's her responsibility to report your stay, she does not necessarily need to accompany you to the immigration office. You can also register online using the provided TM30 link, where she can fill out the required information, and then print a reference to bring to immigration if needed. Always ensure to have your passport on hand.