just to add…
almaden community center’s tuesday night badminton is from 5-9 and is separated into two time slots: 5-7 and 7-9. you must sign up for 5 weeks at a time to be guaranteed a spot in one of the two time slots. drop-in fee is $5, and even then you aren’t guaranteed a spot, and drop-in only applies to one of the time slots. max occupancy for any time slot is 12 guaranteed spots, 2 drop-ins.
at camden community center, drop-in is also $5 i believe, but people are never there and the nets are very poorly set up. there’s no official height marker on the poles and they sag towards the middle. i’ve been there three times and no one was ever really there.
evergreen valley college also has drop-in badminton fridays from 6-10pm for $5 but the court system is very poorly regulated.
campbell community center has drop-in and it’s pretty great. i haven’t been there in a couple months, so i don’t know if the same people go but it’s never really crowded. all the info you’d need is here http://www.ci.campbell.ca.us/Recreation/fitness/dropin.htm