I have never discovered software via github and I doubt that many do quite frankly. People discover software via social channels. People who have heard of a piece of software and want to learn more about it usually do so via their search engine not directly via github.

Having discovered it account creation DOES present a barrier to entry for those that want to contribute but nowadays many such services allow one to sign in with a social service. For example Gitlab.com allows one to sign on with google, twitter, salesforce, github, or bitbucket. Likely reducing the barrier to account creation to 30 seconds via oauth or 1 minute to create a new account via email.

I don't find it credible that people who want to contribute hours to hundreds of hours of work will be liable to be put off by the need to do half a minute of work.