Reporting a Bugs
You can first check for existing bugs and report those you've found concerning dar or libdar. To open a bug this way, you will need to have a user account at sourceforge. This is necessary to have a contact to request additional information to, in order to fix the reported bug.
But better firt qualify the problem you meet, asking support on the dar-support mailing-list so you will probably get a answer quicker and will not have to fill a bug report on sourceforge (this will be done for you).
Feature requests are listed here. You can vote for the one you want to be implemented (the ones that interest most people get implemented first) or add your own one.
Provide or use a patch
You are free to provide a patch for dar and/or libdar. You are also free to use submitted patches but pay attention to their status and use only validated ones for the security of you data. Alternatively you can submit pull request on github.
General discussion about
The dar-discussion mailing-list is open for discussion about dar/libdar and other software relying on libdar when this is not related to support requests or bug report.
Contact the author/maintainer
However, just in case, you get email address from the gpg key used to sign the releases and also find it from the AUTHOR file present within the source code.