When using multiple accounts with GitHub, it may be necessary to commit changes to private repositories which may not accessible by all accounts.

This is how to generate the key and add it to the SSH agent; then add it to the GitHub account too.

ssh-keygen -t RSA -b 2048 \
    -f ~/.ssh/github_aa -C "marco@alertavert.com"
eval "$(ssh-agent -s)"
ssh-add ~/.ssh/github_aa

The original solution was here.