Complex sentence: Matica Climbing Club (shortly or just), with its office located in Ljubljana, is by the number of members the largest mountaineering club in Slovenia and a member of the Mountaineering Association of Slovenia.
Split into simpler sentences: Matica Climbing Club (shortly or just), with its office located in Ljubljana, is by the number of members the largest mountaineering club in Slovenia. It is a member of the Mountaineering Association of Slovenia.