Guitar Rig is an effect plug-in. It doesn't have the ability to produce a sound source - but it does have the ability to effect an existing sound source.
You can place GR as an insert plug-in on an audio track.
You can also place GR as an insert plug-in on a SI track.
So, try inserting an EXS24 on a SI track and select a guitar sample instrument from the EXS24. Now place GR as an insert on that track.
Play a few notes - change some of the settings on GR to taste :)
You can send from as many tracks as you wish to a Bus. On the corresponding Aux track place Guitar Rig as an insert plug-in and adjust the send values from the other channels to create some great effects.
Using an insert on a Bus that is being sent to by say 10 channel strips is more efficient than placing 10 instances of GR on 10 channel strips.
There are so many routing possibilities in Logic ---- WOW! It really depends on what kind of effects you want to achieve in your music!
the Logic 101 video explores some of these:
You may also be impressed with Logic's side chaining abilities (also covered in the above tutorial i think - do check out the contents first) and is taken to a more advanced level in Steve H's TNT series 1 (December videos):
Let me know if you have any further questions :)