Yeah the a+s is the right set uid command. And if you look into groups and start seeing people talke about a hex number like 644 or any other applicable number, it's just the equivelant to using chmod ugoa+rwx where u=user g=group o=other and a=all as isajera said and the read, write and xecute tags. Here is how I remember the hex numbers sorta.
The first number is equivelant to u the second g and the third o, so if you want to set a files permissions to say r--rwxr-- it would be either;
chmod u=r,g=rwx,o=r filename OR
chmod 474 filename
Hope this helps some if any...