Can I send a group message to about 100 people on my iPhone?

No, I cannot. I don't use one, and from what I understand, many of them that do provide this service require the recipients must have the same service on their devices. These are the kinds of things that email is made for, and even that should not be sent to that many people at once. Most mail clients would identify it as spam and block it. Large groups of recipients is one way that junk email is identified in mail clients.

Susie-que wrote:

Thanks Chris. Can you recommend one?

Look into GroupMe. https://groupme.com/en-US/

-AJ

Not with the native iOS Messages app, or with SMS. You may be able to do it with a 3rd party messaging application.

You're welcome, and the same for you.