I do not know a way to add someone to multiple groups.
Reviewing through your inquiry, though there is not an OOTB way to add a user to multiple social groups at a time there are a couple of options available:
1. If you are interested in developing a custom tool to allow for this action the Jive REST API v3.14 → Member service should provide a starting point. Additionally, I would recommend reaching out to the Developer space here in the Jive Community if you are interested in pursuing an API option as there you will be able to collaborate with other developers on your potential customization.
2. If you would like to see this type of feature added to a future release I would recommend submitting an idea to the Ideas for Jive space where our Product Management team reviewed through top rated ideas when considering features for future releases.