Online email mailing list options?

Not sure if this is the right/correct/best forum, but I am asking about software... just not client-side software.

So... we have a group of 20-some-odd friends who hang out, and get together and do things.

Since none of us actually work together or even live near one another any more, a lot of the coordination happens via email.

Invariably, 1-7 people are "left off of the list" any time someone initiates a "let's do [xyz] this weekend" email... so those people have to be added in manually (when people realize that someone has been left off).

A year or so ago, I recognized this need and attempted to fill it (at least partially) by compiling a "master list" of names and email addresses in Outlook, and then sending the "mailing list" out to everyone as an attachment.

This works fine for the people using Outlook (about 1/3 of us).

I copied the names and addresses into a "group" in my Gmail, so I can also initiate a group email that way, too. But I don't believe I can send this out to all of the other people in the group (like I did with the Outlook mailing list).

Ideally, what I'm looking for is a list manager that I can post/host online, and people just go to the link. It would be sort of like a de-listed YouTube video... it doesn't show up in a Google search, but if you have the URL you can see/get it.

Ideally (lol), it would also work across platforms (Outlook, Gmail, Yahoo, and Hotmail) are the most common email providers in question.

At this point, I'm thinking a Facebook group might be best, but a few are not on Facebook and I was hoping to find a more email-centric solution.