Why does the Teiko basketball club have over 100 members?
The second/third string in Teiko play matches against weaker schools. In one of the OVAs (Tip Off) Kise (when he first joined) and Kuroko help the second string in an exibition match. 

Also, first years and even second years would not usually make first string right away in a bigger school, but they still need training so that they can possibly join later.

They could just have joined to play/practise basketball. Remember that it's a club not just a team. I doubt that anyone would even attempt to join if they had to make first string. It's pretty clear playing in matches is a privilge for most people, not an expected outcome.