I have a file server (Pi w. Raspbian) with a project disk setup in my LAN.
I need to give FTP-access to one project folder at the server with files and folders, both created through LAN and uploaded/downloaded via FTP readable by anyone, both local and ftp-users (with rights to it of course).

I've created a "ftpuser" and a homefolder at the server, mounted /mnt/WORK/project-X in it.
Set up a FPT using vsftpd.

How should I set this up regarding to permissions, groups, etc?
Any way I try I either can't create folders and files from one of the users or they are not readable by some others.

You probably need more info regarding my setup so please ask.

It sounds like you want a umask of 022 - look in vsftpd.conf.

1> chown -R ftp:ftp /home/ftp_username/
2> chmod 775 /home/ftp_username/
3> if you have selinux Enforcing mode then you should change it to Permissive or disabled.Otherwise use this :setsebool -P ftp_home_dir=1
4> now you can able to login using specific user credentials and now you can create file and dir under it.
5> also you can change other $HOME dir by using this methods :
# vi /etc/vsftpd/vsftpd.conf
chroot_local_user=YES
chroot_list_enable=YES
# (default follows)
chroot_list_file=/etc/vsftpd/chroot_list

# vi /etc/vsftpd/chroot_list
Ftpuser name

Hope it will solve your issue.