What to use to organise a "weminar"?

[Lucile Fagueyrac]

Hi,

I'm supposed to organise an online workshop next week.
I would just like to brodcast what is happening on my screen to up to
15/20 people.
I don't even really need video or audio, just a screen broadcast would
be enough.

So far, my tests of jisti are not very conclusive.
People keep telling me to use skype or google hangouts / other
proprietary services.

Does someone have a better idea? What would you do if you were in my
shoes? The viewers are not using Free Software, and are both on Mac and
Windows.

Thanks!
Lucile