You’ve probably ment that one user is in a more than one workgroup. Because if a user is in two queues of the same workgroup he will still automatically login to that workgroup. If the user belongs to more than one workgroup, then he has to manually login to a desired group. I think this was by design, so there is no option to change that.
I will quote my statement from this doc about the Fastpath: http://www.igniterealtime.org/community/docs/DOC-1876
First and most important to know! Fastpath is not in the active development now, as the most of the former Enterprise plugins. Jive Software has released all commercial plugins (except clustering) as Open Source and stopped their work on them. So far only one community member is doing some customization development. So, most of the feature requests and bug reports won’t get any attention. If you plan to use it in a serious production environment, then consider choosing other solutions.