Hi Sven Lindenhahn -
There is a feature built into Jive called External Contributors, where you can have external users invited to secret or private social groups who may not otherwise have access to your Jive instance. This is not an extra "Plugin" but is a built in feature of the Jive platform and is available in Jive 7, Jive Cloud, and will be available in the upcoming Jive 8 release.
There is some documentation on this feature set here:
Is this the feature that you're referring to?
Jive 7 and 8 instances can be migrated over to Jive Cloud instances. You can file a support case when you want to get this upgrade process started and our support team can help initiate the next steps for planning that.
Let me know if this helps or if you have any additional questions.