/etc/group what's the difference between....

[datadan]

I am haveing some trouble understanding the difference in ramifications between useradd -G and useradd -g

I see the differences:
in etc/group
test:x:775: test1, test2, test3
test = group name
x = password
775 = group ID
test1, test2, test3 = ? (This occurs with -G)

in etc/passwd if I add the users using useradd -G test test1
test1:x:782:782::/home/test1:/bin/false
test2:x:783:783::/home/test2:/bin/false

in etc/passwd if I add with useradd -g I get
test1:x:782:775::/home/test1:/bin/false
test2:x:783:775::/home/test2:/bin/false
but test1, test2 are not added to the end of /etc/group test?

There is something I am not understanding and the man pages are not particularly helpful here. Maybe someone can point in the direction of a particulary helpful document somewhere.

Much obliged in advance for the help.