You need to set the "mode" parameter to include the group authorities, in addition to the user authorities.
mode_t fmode = S_IRWXU | S_IRWXO | S_IRWXG;
S_IROTH - General read permission
S_IWOTH - General write permission
S_IXOTH - General execute permission
S_IRWXO - Is all of the above
If you need to do it after the file is created, you can use CHGAUT. I had to use this for files created by a qsh('jar...').
Ah, Dale's comment made me realize you said "change this" as in change the existing file, where as I assumed you ment you wanted to modify the program that was creating your CSV. So CHGAUT is easiest. If you want to use the API to do that, it is:
chmod( ifsFileName, newModeFlags)
D chmod PR 10I 0 extProc(*CWIDEN :'chmod')
D path * Value OPTIONS(*STRING:*TRIM)
D mode 10U 0 Value