It was likely one of two things...either A) an off duty F/A or non-F/A employee was traveling on their company passes and was simply lending a hand(that type of thing is encouraged)...or B)it was an actual working F/A who had done what is called a jetway trade. In short a jetway trade permits an "off-duty" F/A to step-in and work a single leg for another F/A under certain circumstances. We are expected to be in uniform in these instances however, provided we are contractually legal, there is no FAR that prevents us from working in "civvies". When in doubt just take note of the following...if you see the person in question perform the safety demo, take drink orders and serve beverages...regardless of how they are dressed...they are an F/A. However if all they do is pass out the peanuts...they are most likely simply a "helper".