Sorry to hear you are not able to join an external social group. As a next step I would recommend submitting a case in your customer group here in the Jive Community where the next available support engineer can investigate directly in your instance. When you submit the case is you could also include the following information, this should assist in their investigation.
1. Is this limited to a specific social group?
2. Are other users also experiencing this behavior, or does it appear to be limited to your user?
3. A link to a social group where you are experiencing the behavior.