what exactly is this club?
An English pop group
S Club 7
S Club 7 were an English pop group from London created by former Spice Girls manager Simon Fuller consisting of members Bradley McIntosh, Hannah Spearritt, Jo O'Meara, Jon Lee, Paul Cattermole, Rachel Stevens and Tina Barrett. 
what does the s in s club mean A theory is that the group is so-named because "S" is the first letter of the word "seven".
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S Club 7 [SEP] Introduction
S Club 7 were an English pop group from London created by former Spice Girls manager Simon Fuller consisting of members Bradley McIntosh, Hannah Spearritt, Jo O'Meara, Jon Lee, Paul Cattermole, Rachel Stevens and Tina Barrett. The group was formed in 1998 and quickly rose to fame by starring in their own BBC television series, "Miami 7". In their five years together, S Club 7 had four UK number-one singles, one UK number-one album, and a string of hits throughout Europe as well as a Top 10 hit on the US Hot 100, with their 2000 single "Never Had a Dream Come True".