Master Aleksandr Ruslanovich :
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.
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.
In general, a situation like this calls for a gentle but firm hand, steering them towards some loaner garb for the short term, and a teacher (or at least a merchant) for the longer. The main danger is coming off as scolding, which can drive someone away -- you need to get across the point that we *do* have some reasonable expectations of folks, while being as helpful as possible. Exactly how you handle it is going to depend on the person involved, and being sensitive to how they react.
Lady Emmanuelle de Chenonceaux :
As Baron, if already introduced, I would approach them. If not I would find someone to introduce me. I would greet them, find out who they were, what their interests were. Proclaim that I need someone with just that set of interests to fulfill a position in court (cupbearer, treasury guard, messenger, spy, or whatever depending on their willingness and interests). I'd inform them, however, that court requires appropriate garb, but that all is well because I have just the person to sort them out. Then together seek out whoever is in charge of gold key and leave them in their hands. Later I would seek out someone who sews and direct them in their direction.
Mistress Gwendolyn of Middlemarch :
If I could find someone who knows the person, and that someone is willing to talk to the person, then I would suggest that the someone have a chat with the person in the very near future and possibly take them to visit Gold Key to see if there is any loaner garb at the event.
If no one knows this person or is willing to talk to this person, then I would introduce myself to them and ask about their interests in the SCA. Then I would point out that some attempt at medieval or Renaissance clothing is expected at an event. If he or she still looked friendly, I would expound on the virtues of tabards and t-tunics. Then I would tell him or her that we would appreciate it if he or she would make the attempt to wear some type of garb. If they still looked friendly, I would offer to accompany them in search of loaner garb.
Lord Kali Harlansson of Gotland :
On the other hand, if I've been seeing them at more closed events where they evidently paid a site fee and otherwise consciously chose to come in, I'd be less likely to come straight over. I'd probably ask around a little first, see if anyone knows who this person is, whether anyone else has ever seen them in garb (maybe I've only seen them coming to or leaving the event), whether anyone else had already talked to them in the manner suggested above. I'd likely still approach them and engage them in talk, but just what tack I'd take would depend on what I'd learned from the asking-around stage.
I would likely do things a little differently as Baron. If they seemed shy and genuinely unaware, I'd worry lest having The Baron Of Carolingia approach them might be intimidating (gentle and lovable though I am); I'd probably ask a known responsible and discreet friend (member of entourage?) to go and try my first approach. On the other hand, the more it seemed that they had already had some exposure to our mores but just didn't care, the more likely I'd be to approach them in person - still open and friendly, but letting them know that collective appearance really does matter to us.
I can't give a clearer answer, it would depend a lot on the situation.
As Baron I would be a bit more circumspect and have someone else approach them and introduce them to gold key and mention that the Baron and a few others are happy to teach. As Baron a comment on garb could be more intimidating, and thus I would avoid making it.
Shi Hua Fu and Lady Yelizaveta Medvedeva :