To enter a ROOM meeting venue as an invited guest, no contact information is necessary, although a guest name is desirable (name specifics are entirely your choice) to label/associate your video cutout to you while in the meeting.
Inside the room venue, you'll notice that every participant has a name associated with his/her video cutout. Their respective names will appear momentarily or when your cursor hovers over their cutout (when their respective body or encircled head cutouts are in view) - for easy identification.
Another reason to have a guest name is to populate the list of attendees. When accessing the Invite/Attendees icon on your screen's lower right, a popup box will promptly display revealing all guests in the room (as well as any number of available seats). This might be important to the meeting's organizer, and might be an issue if several people enter the meeting allowing ROOM to assign them cryptic default names (which look like: Guest-1, Guest-2, etc.)!
Experience leads us to assert how important it is to have a name associated with your video cutout, especially in meeting venues with large gatherings. There are no restrictions concerning the characters you need to enter, so feel free to express your identity in any way you feel fit!