See also the official documentation:
Help Center > Group Members Manual > Managing group topics and messages > Splitting topics

In the Topics View, go to one messages in the topic that you want to split off from that topic. The new topic will begin with this message. 

Click in the More to open a drop down menu for the message you wish to split off. 

Select Split. In the box for new subject, add your new topic (the subject for that topic).

All messages newer than that one will now be in the new topic. You can use merge to move other messages around to the right topic.

[NOTE: There's still a bug in the system that Mark is aware of.  It won't let you split off a post that's on top if you've sorted by reverse date (newest on top.)  You have to change the date order to do the split or wait for another post to go to the top.]