Currently there's no UI to check for all invitations to collaborate in projects, but Github API has an endpoint for that.
You need a personal access token with repo:invite rights in order to view the list of your invitations.
curl -H "Authorization: token YOUR_PERSONAL_ACCESS_TOKEN" \
https://api.github.com/user/repository_invitations
curl -H "Authorization: token YOUR_PERSONAL_ACCESS_TOKEN" \
https://api.github.com/user/repository_invitations | \
jq '.[].html_url'