Wouldn't the easiest option be to create a new "discussion" for each category? For instance, if I am going to post a recipe for pie and their is no category yet, I would start a new discussion titled "Pies" and post it there. Other categories ideas, "Side Dishes", "Appetizers" or "Sandwiches & Salads", "Desserts", "Yeast Breads", "Quick Breads", etc. Just some ideas. Then, if the group owner prefers, he/she can categorize them as desired.