Multisite or Subdomain with same users
We currently have a site - www.example.com
I'd like to setup classipress on buyandsell.example.com but maintain the ability to have our users be able to log into either site - is this possible? I've searched but nothing I've found has really answered this question.
Secondly, if I'd like to keep www.example.com
and then add, as a multisite, www.example.com/buyandsell
and install classipress on that section, is that possible (and/or a better option)?
Thanks in advance!