Directory Rights

I'm using linux for years, but the user rights still arent clear at all.
I've already read many articles, but it's still not clear how its works.

Here is my example:
i'm logged in using my username 'ant'

I want to write into the convert directory as shown above
drwxrwxr-x+ 4 hts video 4,0K nov 12 08:40 convert/

ant@nil:/storage/serverdatas/tvheadend/recordings/convert$ touch aa
touch: cannot touch 'aa': Permission denied

ok. So i did a check
ant@nil:/storage/serverdatas/tvheadend/recordings/convert$ members video
vdr s_ftp ant

ant@nil:/storage/serverdatas/tvheadend/recordings/convert$ members hts
hts s_ftp ant

So i think ant is a member of video group. the convert directory is owned by this video group and writable for the group members.
But its not. If i enable to write for everytone it works, but i't would be nicer to make them writable for users who really needs them.

Can anyone explain where i did the mistake ?