Ignoring the header (which you can tack on later):
awk -F, 'NR > 1 {print > $2}' use_rep
which will print each line to a file named by the second column:
~ head *[0-9]*
==> 100K+ <==
86440,100K+
116858,100K+
22222,100K+
38906,100K+
==> 200K+ <==
22565,200K+
7453,200K+
==> 500K+ <==
885,500K+
==> <100K <==
10762,<100K
To put the header, maybe something like:
awk -F, 'NR == 1 {header = $0; next} # save header, skip this line
!a[$2]++ { print header > $2 } # print if second field hasnt been seen before
{ print > $2 }' use_rep
Result:
~ head *[0-9]*
==> 100K+ <==
user_id,rep
86440,100K+
116858,100K+
22222,100K+
38906,100K+
==> 200K+ <==
user_id,rep
22565,200K+
7453,200K+
==> 500K+ <==
user_id,rep
885,500K+
==> <100K <==
user_id,rep
10762,<100K
70524,<100K
