The company I am with has acquired another company. My company has a complete domain with AD and O365. The other company has no AD but they are on O365. I moved them from their O365 tenant to our O365 tenant. I am trying to figure out a way to get Security Groups created with users from both companies. We will be merging departments as changes are made at both companies. I need to find out how to get combined employees in the new Security Groups.
I guess you mean on-premises security groups, right? You will need to create objects representing the users in your AD, which is easily done by exporting via PowerShell or the Graph API and importing to AD. Once you have the object created, you can "match" them against the cloud ones: http://support.microsoft.com/kb/2641663
And then you can work with the users as if they were your own :)