We have a single ANTLR v3 contributors file in the repository that contains the contributors license. Anyone that wants to be a contributor, can simply fork antlr/antlr3 from github, add their name to "sign", commit, send a pull request and then they are in the system. Any future pull requests can be merged with a quick check on their name. The contributors file can also be augmented with their signoff him during the first pull request.
This is on a per project basis.
On a related problem with pull requests. As Sam Harwell pointed out to me, github has a pretty serious problem when it comes to forked repositories:
Pull requests are still being merged with the syntax "Merge pull request #23 from parrt/master", which will then propagate to and mess up the issues of everyone who merges antlr/antlr3 into their forks.
This applies to the issues that people have in their particular forks, not with the antlr/antlr3 repository. Consequently, I often do a manual merge for pull requests.