IM gateway and/or s2s for a group chat on private openfire server

Here’s my use case. I want to setup one or more group chats for our members of a non profit organization.

We’ve tried IRC, but many had issues with firewalls. Most of us have a gmail account, so using gmail’s gtalk interface should work.

The idea was to set up openfire on our server, set up a group on that server, and make it so that people can either connect to our the openfire server or bridge the server with google gtalk /

I am a little bit lost at what I need in order to achieve this. Do I need the IM gateway plugin and/or do I need to setup s2s ? What do I need to to with the DNS ? Should I be concerned with permissions with regard to accessing that group ?

It would be great if someone can give some sort of overview of what is needed, and maybe list common pitfalls.

Thanks for your help.