Goldy Gopher welcomed guests as Northrop Auditorium reopened Friday after a three-year, $88 million renovation, with a performance by American Ballet Theatre. Architects from the Minneapolis firm HGA shrank the auditorium from 4,847 to 2,700 seats and added a new lobby, balconies, wider seats, almost twice as many bathrooms, a rehearsal space, dressing rooms and a new, 168-seat theater for movies, recitals and lectures. “Giselle” will be performed again at 8 p.m. Saturday and 2 p.m. Sunday.