As an individual, I would approach the person and direct them towards
Gold Key.  If they had no interest in dressing in garb, I'd probably
just ignore them.
<p>
If I were baron, I would start by talking to the folks manning gate. 
One of the few criteria of participation in SCA events is "an attempt at 
medieval clothing".  If someone showed up in modern clothing -- and had 
no obvious change of clothes with them -- I would encourage the 
gatekeepers to explain the rules of participation, and point the 
newcomer towards someone who could provide them with clothing.
<p>
After that, I would find a Gold Key officer and point them towards the 
person, asking that Gold Key try to help them out.  In some cases, like 
an event at a borough, I might approach one of the borough 
representatives first.
