In unix inspired operating systems one can see whose groups they belong to by using the id command:

[pflorido@pptest-iridium ~]$ id
uid=6312(pflorido) gid=34000(clusterusers) groups=34000(clusterusers),6300(hep),6500(npusers),46300(ppguests) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[pflorido@pptest-iridium ~]$ touch testgrp
[pflorido@pptest-iridium ~]$ ls -l testgrp 
-rw-r-----. 1 pflorido clusterusers 0  6 feb 18.09 testgrp

The gid field shows the primary group, that is, the first group you belong to and with which one's files will be created.

If one wants to switch to another primary group so that the files are created with a different group name, one can use newgrp:

[pflorido@pptest-iridium ~]$ newgrp hep
uid=6312(pflorido) gid=6300(hep) groups=6300(hep),6500(npusers),34000(clusterusers),46300(ppguests) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023bash-4.1$ touch testnewgrp
bash-4.1$ ls -l testnewgrp 
-rw-r-----. 1 pflorido hep 0  6 feb 18.27 testnewgrp

Note that group switching starts another login shell, so you can use exit to go back to your previous group.

bash-4.1$ exit
exit
[pflorido@pptest-iridium ~]$ id
uid=6312(pflorido) gid=34000(clusterusers) groups=34000(clusterusers),6300(hep),6500(npusers),46300(ppguests) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023