I am not sure if what you are looking for exists. In these festivals you will usually have three groups. You can try joining one of them:
- Artisans and food stands. These will be working or cooking on whatever they do. They will be dressed up, medieval style.You would need to be an artisan or maybe be willing to cook or sell food. You need to find out who does this. This usually provided by local businesses or small associations. If you are an artisan, you can usually enroll to be present.
- Than, there are theater or performing arts groups hired for animation. They usually do street performaces. Eg.: Juggling, or theater role playing. These will also dress up. You can try to find a theater group and let them know you would like to participate. Often the theater groups can be local amateur groups. That would probably be easier to join.
- Last but not least you get the visitors.
Visitors can also dress up. You can either bring clothes from home or rent them at the festival. You will be enjoying the spirit and its probably the easiest way.
Often these festivals rely on volunteers for some management work in place> Welcoming visitors, etc.
I have done some search and found out that for Santa Maria da Feira they actually have this type of role as well:
You can enroll and wait to be accepted.