The first one (work out) is an invitation to exercise, most likely. You'd use this if you're at a gym and there's someone you want to join your group while exercising, for example. The second is very different- (walk out) means to leave, by literally walking out. It can be used to just ask someone if they want to leave the building/ place with you. Alternately, "walk out" can be used to indicate that you are leaving in anger.... usually dramatically or in anger or frustration. You'd use this (much less likely BTW) if you are mad in a restaurant and want to invite others to leave with you, because you're mad about the service. If you walk out of a job, for example, you are quitting suddenly, in anger.