Gutenberg

Hi there, I'm runnting btsync on a Raspberry pi. When I run the command ls -l concerning a file created by btsync I recive the following -rw-r--r-- 1 root root this means user "root" is allowed to read/write, the group "root" can read and all other users can read the file as well. However, I want to gain write access by a different user, which is not in the group "root". The ouput of the command ls -l should then be -rw-rw--rw-- Can someone show me a way to do so?