Jan 26 2021 06:41 AM - edited Jan 26 2021 08:03 AM
@Mary_Yvette Hi, in addition to the previous reply this (Together mode) wasn't possible a couple of months ago being on GCC or using Teams free, but everywhere else I think. I have a Teams free org. and when scheduling a meeting right now I can see the option (greyed out though as I haven't sent out any invitations). The same goes for my personal Microsoft account using Teams, it's there.
So I would say Together mode (in other words the new meeting experience) seems to be available for all subscriptions.
As for the guests and externals it won't work for an external user as far as I know. Only with a guest user as the latter has a account in Azure AD. Haven't done any test with an external user though. Maybe I have to do that now :)
And just a heads-up that guest access in general are controlled by your IT admins. And Teams meetings follow the "Meeting options" set per meeting (end-user controlled) or by using Teams policies (IT admin controlled). That goes for guests, federated, anonymous users etc. meaning the lobby settings and so on can be adjusted.
Btw, see this for online experience (web) Now in public preview: Together Mode and Large Gallery for Web Meetings - Microsoft Tech Community
Hope it helps!