Complex sentence: Over the next six years, interest and attendance grew until a team was formed; the Rio Guardians was founded on January 4,1992, becoming the first American football team in Rio de Janeiro.
Split into simpler sentences: Over the next six years, interest and attendance grew a point at which it made sense to form a team: the Rio Guardians was founded on January 4,1992. The group became the first American football team in Rio de Janeiro.