I'm happy to share my criteria for being alone. Of course, it is not set and definitive for everyone. I hope it puts things in perspective for anyone who may feel the line of alone versus loneliness is blurred.
From my perspective, being alone can absolutely be a feeling that arises in the presence of others. It is possible to stand in a crowd or group and feel that your presence or lack of does not make a difference to anyone. You may feel alone if you find comfort in removing everyone from your presence or immediate environment. It may also be the case that you are unwilling to trust, share feelings or belongings with others. Though unrelated, the slightest display of disinterest in you or a subject matter you present confirms your feelings.
If you identify with being alone, please take the necessary steps to liberate yourself from that feeling.