Multiple developers with local access to the repository may share it using a common group. To do so, create a new group and add all developers to it. Run "chgrp -R sharedGroup repdir ; chmod -R g+s repdir" for the shared group and the repository directory. Now, on local access to this repository everybody will create files with the appropriate group setting. However, the developers will need to set a liberal umask before using svn (like "0022").
If somebody resists to do so, there is still a brute-force solution: fix the permissions with a post-commit script. However, this is an "unsound" solution and may lead to ALL KINDS OF PROBLEMS. MAKE SURE THAT YOU ARE AWARE OF THE POSSIBLE CONSEQUENCES BEFORE YOU OPEN THE PANDORA BOX. See Debian BTS for details. When you damage your repository, don't blame me and remember that there is "svnadmin recover".
Example 2.2. post-commit hook example
#!/bin/sh # POST-COMMIT HOOK # The following corrects the permissions of the repository files REPOS="$1" REV="$2" chgrp -R sharedGroup $REPOS # replace sharedGroup with your group chmod -R g+r $REPOS chmod -R g+w $REPOS