The word bullying brings to mind hitting or shouting. In practice, the harder-to-see forms are more common.
No reply when you speak to them. One person left off a group message. Taking turns leaving someone out.
It Lasts Because It's Hard to See
With no obvious action, people around don't notice. The ones doing it can say "we didn't do anything."
The person on the receiving end suffers from having no proof. Being told "you're overthinking it" is what stops them speaking up.
What You Can Do When You Notice
If someone is missing from invitations, speak to them yourself. Checking whether everyone got the same message is another way.
If it's happening to you, note down when, who, and what. It helps when you ask for help. Keep screenshots too. Check now and then that you aren't the one leaving someone out.
Look around once for anyone left out of plans or messages, and speak to them.
At Digital Kodomo BASE, we value paying attention to the forms that are hard to see.
Check Which is true about exclusion?