You can base part of a file's permissions on part of its existing permissions. To do this, instead of using r, w, or x after the operator, you use the letter u, g, or o. For example, the mode
o+g
adds the permissions for users who are in a file's group to the permissions that other users have for the file. Thus, if the file started out as mode 664 (rw-rw-r--), the above mode would change it to mode 666 (rw-rw-rw-). If the file had started out as mode 741 (rwxr----x), the above mode would change it to mode 745 (rwxr--r-x). The - and = operations work analogously.