I ran into a several relatively trivial issues with adding users to the Members and Owners groups in SharePoint 2013 Beta
My administrator user does not have a Site Settings option under the gear icon at the My Site. However, this was available under my Home site ("/"), so doing some URL mangling, I was able to get the My Site Permissions page by going to /my/_layouts/15/user.aspx
Note that "All Authenticated Users" is now called "Everyone"
The people picker doesn't look like it's working. I get a "Sorry, we're having trouble reaching the server". However, I am able to add users (ie: Share, in SP2013 speak) if I enter the complete name in the picker.
Finally, the "Send an email invitation" setting is checked by default and only accessible after you expand Show Options. That is annoying.