With item group entry providers you can add your item to a certain item group. These entry providers can be found in a data pack under the item_group_entry_provider folder in a namespace. For the vanilla item groups, there are plenty of item tags available you can use to add your item to existing subgroups. These item tags are found under the item_group folder in the item tags in the minecraft namespace. 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 stack entry provider with the specified item. Each entry also has optional fields to fine-tune the visibility of the entry. Note that if two item stacks with the same components are added to the same item group, it won't add that second item stack and will log a warning.

{
  "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"
    }
  ]
}

The stack visibility determines where in the creative inventory the entry is visible.