void OnUserGroupAdded(string player, string groupName)
{
    if (Interface.Oxide.DataFileSystem.ExistsDatafile($"{Name}/commands/{groupName}") == true)
    {
        Puts("Exist!");
    }
    else
    {
        Puts("NO!");
    }
}

CommandsPluginData commandsDataFile;
class CommandsPluginData
    {
        public int inventorySlotsNeeded;
        public List<string> commands = new List<string>();
    }

void OnUserGroupAdded(string player, string groupName)
{
    try
        {
            commandsDataFile = Interface.Oxide.DataFileSystem.ReadObject<CommandsPluginData>($"{Name}/commands/{groupName}");
        }
    catch (Exception e)
        {
            PrintError(e.ToString());
        }
}​

Ultra

The first one should work as I use it
Interface.Oxide.DataFileSystem.ExistsDatafile(path)

Just check if path is correct. I think that the final file should be without .json extension. So the path should be "myfolder/myfile"

Thank you for your answer! It's now working, I made a mistake in my code :)