How do you join a group?

I am new to Skydrive and wish to use the group function to share files with a project member at another company. I created a group and invited her to join. She can see the group but we can not find how she becomes a member. The invitation was only to view the group. She is not listed as a member even though she clicked on the email message to view the group. She can not upload files to the folders I created. What am I missing?