This section of the FAQ should answer your question sufficiently.
4.12 - Majordomo doesn't seem to work with sendmail 8.9
The new security features of sendmail don't allow :include: directories to be group writable. Unfortunately, by default these directories are group writable with Majordomo. If you have this problem you will see errors from sendmail like "Cannot open /path/name: Group writable directory" and "aliasing/forwarding loop broken".
One solution is to add:
in your sendmail.cf and restart sendmail.
The other method (and generally the recommended one) is to remove the group-write bit on the lists directory and any list files. Make sure also any parent directories to not have the group or other write bit set. If Majordomo is working correctly having group write permission is not necessary. However, some people find it convenient to have group-write access so users can be put in the majordomo group and not need root access all the time to work on majordomo.