Complex sentence: The Italy national rugby sevens team have occasionally competed in the IRB Sevens World Series since the series' was introduced in 1999, although they do not participate in every leg.
Split into simpler sentences: The Italy national rugby sevens team occasionally compete in the IRB Sevens World Series, and have done so since the series' was introduced in 1999. Although they do compete in the series, they do not participate in every leg.