linux - Why cant a group member modify a file in a group he is part of?

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-edit- i figured it out. I needed to
use adduser username group
instead.

I added a user with the command

useradd -G myapp_user
            newusername

then i
changed the group on the public folder that i want the user to access. Here is the line
using ls -l I see the group has been
set.

drwxrwxr-x 3 root myapp_user
            4096 Jul 9 19:13
            public

I cd into it
and do it again to and i see files like the below with the group. I also see rwx on both
the public folder and on the content inside the
directory.

-rwxrwxr-x 1
            root myapp_user 4403 Oct 10 2007
            info.png

Then i login
as the user and CD into the folder i wrote touch a and i got a
permission error. I cant add, delete or do anything even though i see 775 is the
permission. I also tried useradd -G myapp_user newusername to
find the user is already part of the group

what
am i missing? am i suppose to flush something before it takes effect? restart something?
why cant the user modify anything in the public
folder?

using putty and winscp. But right now
just putty.

-edit- i wrote
id newusername and got the below. Why isnt the groupname
included?!

uid=1000(newusername)
            gid=1000(newusername)
            groups=1000(newusername)

Answer

use adduser username group
instead