@misc{cryptoeprint:2007:186,
author = {Jens Groth},
title = {Fully Anonymous Group Signatures without Random Oracles},
howpublished = {Cryptology ePrint Archive, Report 2007/186},
year = {2007},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.