One of the most important Communities feature (compare to Portal) is availability of Chatter, where Salesforce user can collaborate directly with their Partner user directly via Chatter.
If you just setup Chatter in Communities and found that Communities user cannot see Salesforce internal user in Chatter, this is because security issue related to user sharing. There are few ways to enable this:
1. Sharing Setting
By default Organization-Wide Defaults for User object for Default External Access is Private, this is why partner user will not able to see internal user. If you change this to Public Read Only, this will allow all partner user in any communities to see all internal users.
2. User Sharing Rules
Create new sharing rule for Portal user to have read-only access to internal user. You can determine which portal roles (and subordinates) able to see which internal user or external user, by Role or Public Group
3. Manual Sharing
This process to manually share each users visible to which Partner role. Go to User detail page and click Sharing button, then specify Partner users (by Role or Public Group or All) able to see that particular user. In screenshot below, Steve is the only user shared to Partner using manual sharing.
Note: if you do not see User object Sharing Setting or Sharing button in User detail:
- Make sure you have setup Communities, from: Setup - Customize - Communities - Settings
- Contact Salesforce support to enable it
- Setting Up Your Salesforce Community video on YouTube
- User Sharing video on YouTube
- Getting Started With Communities document from Salesforce