Add a dll dynamically to the application

Why you want to give dynamic reference to solution??? i dont understand your requirement fully, my opinion to load any DLL's dynamically from any physical location You can do as follows,  by make use of System.Reflection namespace you can load any DLL's dynamically,  In that namespace you have Assembly.LoadFrom() method to load a DLL file dynamically, Initially you may load a common DLL / Main Application to get the User Credentials,  After getting your user credentials you may start new Main Thread from different DLL's by using these methods,
Assembly.LoadFrom() - to load the DLL , will Return a Assembly Object,
In Assembly Object we have a method called GetTypes(), this will return different Classes inside,
By using GetTypes(), we may get the classes resides in the DLL in the form of Type, Using any particular type object, we can retrive any methods to start new application,  Type object have got a method called GetMethod() , which will return a pointer to the method, means, will return MethodInfo object, by using this MethodInfo object, we can Invoke any particular method to start own/new main thread for a specific DLL, from that method onwards your New Main Thread will be started.
MethodInfo object have got Invoke() method to run any specific method inside that DLL.