I'm sorry to hear about the issues you've experienced while searching for the user and adding him to a user group. If the issue around adding him in to the user group is also because you're unable to search for him, then I'd recommend performing a user search index rebuild under System > Settings > Search > User Search > Index Tasks. This may take some minutes to some hours, depending on how many users you have, so I would suggest doing this off peak hours.
If the issue persists, please submit a support case in your private Jive Community customer group for direct troubleshooting assistance.
thanks for your help. This worked out for my topics.