Yes, you’ll be able to include the guitar if it provides a line level signal. You may need a guitar preamp for this.

Regardless, I think that you’ll be frustrated if you attempt to do this because there will be a 75ms latency between a “pluck” and output from the FIVE. It’s no big deal if a decades old LP is delayed an additional 75ms, but this 75ms delay will devastate most amateur guitar players. If you regularly play in large halls, you have already learned how to deal with this delay, however, this may not be the case for novice listeners in a small room.

Might actually be “good training” (military for sucks) to get you ready to play in large venues where delayed sound is a given. :-)