As far as I can tell, it is not possible to subscribe to a published/shared .ics calendar (e.g., Google calendar) into my Trumba calendar. I like Trumba a lot, but the reason I subscribed to it was its embrace of open standards. I have exactly one public (Google) calendar that I need to subscribe to. If I can't do it, I will not stay with Trumba over the long term.
If this is possible, then you need to improve your help pages on the subject - I've just spent a couple of hours trying to figure it out and can't (and I'm a computer geek).
I realize you may be concerned about competition from Google calendar, and I understand that concern. But you can't get away with prohibiting people from mixing in calendars not generated by Trumba. And don't tell me that I can export from Google.