Hi Alice,
Go to SHHK website... under the FAQ,it's stated. Also, if you call the staff there now regarding this year reg, they will tell you the same too.
E.g. If there are only 1 vacancy left, someone with who joined for 6 yrs as compared to 1 who joined for 1 yr, the one with 6 yrs will get it, provided both are just normal members.
Committee members get higher chance too.