OS X 10.11 GM Candidate 1 left my Mac unable to boot
You do have a recent backup? If not this procedure will be substantially different.

Backed Up? Use Recovery Mode

If you boot into recovery mode using  Command+R at startup can you proceed with an internet recovery by installing over top of the hosed OS already on the drive?

If that works I would try that first. It should leave your files and apps alone for the most part. If it fails then you may have to reformat the Mac OS partition and install clean using Internet recovery.

No Back Up? Use Target Disk Mode

If you do not have a backup you may have to put your system into target disk mode by holding T at startup and hooking it up to another Mac with a Thunderbolt cable to back up your files.