For new users, you can specify which group they should belong to, both
for ftp-users (if you are using pure-ftpd's own user database) and for
standard linux users (i.e. the "system users").
For pure-ftpd users you use the -g switch to pure-pw:
pure-pw useradd the-username -g ID_OF_THE_GROUP .... (add more
parameters here if needed)
For system users, the useradd command accepts the --groups and --gid arguments:
useradd --groups group1,group2,group3 my-new-user
or
useradd --gid 1234 my-new-user
Hope this helps
-Isak