I found this article at Forbes.com. It does answer my question directly. In summary it says:
Define what it is that you are looking for from the group: connections, information, exposure. To do this you need to know who is your authentic self. Put another way, what is your Brand.
Be discriminant about the groups you join. Only join those that further the goals you have defined. Research the potential groups to find out if they are in line with your goals. Join and try the groups found with your research efforts.
KDNuggets used number of members, growth rate, number of conversations and member engagement in their analysis of data science groups. This analysis can be used to answer if the specified groups are inline with your goals.
The article goes on to give advice about etiquette once joined, but that is beyond the scope of my question.