I don't think that this question has been asked previously on SFSE. Assuming I am migrate to partner Communities from Partner Portal, is it possible to add a partner portal user to my private group in Chatter? Scenario: I have a range of partner portal users that I would like to engage with via Chatter, therefore I would like to be able to set up a private chatter group in my org and add in the partner portal users? If I can add them will they need to log in to the group everytime or will the Chatter group show up in the chatter groups? Is this a better solution to this proposal?
I'd say it's not possible to do this the way you described it. After moving to communities your partners will have access to Chatter but it will be independant from internal organization's Chatter (let's call it Internal Community). Profiles, Groups, Files and all activity everything is visible only inside a community even if you're a member of all communities.
This screen is taken from this documentation page: http://help.salesforce.com/apex/HTViewHelpDoc?id=networks_visibility.htm
So to achieve what you would like to have you would need to add some internal users to partner community. Then creating such private group shouldn't be a problem.