Hi, i had a similar problem, and I think there are several issues:
1) A line break even in the HTML editor for WP translates to a tag.
2) You can’t put a script tag in the WP editor.
3) You may not have replaced NAMEOFMYFORUM with the name of your forum.
The following code worked for me:
<iframe id="forum_embed" src="https://groups.google.com/forum/embed/?place=forum/NAMEOFMYFORUM#!forum/NAMEOFMYFORUM" scrolling="no" frameborder="0" width="100%" height="700"></iframe>
(And yes, NAMEOFMYFORUM appears twice in the URL)
