SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-query-forums

QueryForums_GetForumsByGroup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Telerik.Sitefinity.Forums;
using Telerik.Sitefinity.Forums.Model;

namespace SitefinityWebApp
{
    public class QueryForums_GetForumsByGroup
    {
        public static List<Forum> GetForumsByGroup(Guid groupId)
        {
            ForumsManager forumsManager = ForumsManager.GetManager();

            List<Forum> forums = forumsManager.GetForums().Where(f => f.Group != null && f.Group.Id == groupId).ToList();

            return forums;
        }
    }
}