With item group entry providers you can add your item to a certain item group. It's almost as easy as just adding your item to the entry provider. The unfortunate thing is that these entry providers don't merge by themselves, and this is intended. Instead, there are plenty of item tags available you can use to add your item to. This might change in the future, as you for example currently can't add an item at the very end of an item group. Additionally, some extra provider types exist to handle a few edge cases.

All the entry providers are found in the entries list, where each entry contains a type to use. The entry provider type is determined by the type field, and all types are found in the sidebar. If only an item id is specified, it uses the item entry provider with the specified item.

{
  "entries": [
    {
      "type": "tag",
      "tag": "minecraft:item_group/food"
    },
    {
      "type": "suspicious_effect_ingredient",
      "item": "minecraft:suspicious_stew"
    },
    "minecraft:milk_bucket",
    "minecraft:honey_bottle",
    {
      "type": "potion",
      "item": "minecraft:potion"
    },
    {
      "type": "potion",
      "item": "minecraft:splash_potion"
    },
    {
      "type": "potion",
      "item": "minecraft:lingering_potion"
    }
  ]
}