All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The wealth of investment companies and associations is liable for Zakah (Alms) as one unit. Therefore, the wealth of all shareholders is treated as one unit as far as liability for Zakah is concerned. Narrated Anas: "Abu Bakr wrote to me what was made compulsory by Allah's Messenger (PBUH) and that was (regarding the payments of Zakat): Neither the property of different people may be taken together nor the joint property may be split for fear of (paying more, or receiving less) Zakat." {Related by Bukhari}.
Al-Imam Ibn Hajar (May Allah have mercy on him) said: "According to the reliable opinion of the Shafie school of thought, the property owned by different individuals is treated as one unit/amount in terms of liability for Zakah." {Tohfat Al-Mohtajj, vol.3/pp.259}.
Consequently, the association`s administration should give Zakah on behalf of the shareholders; (2.5%) on all the assets: cash, goods or due debts except fixed assets for these aren`t liable for Zakah. After that, the amount paid by the association is deducted from each member according to their share. However, if the association failed to give the Zakah for any reason then it is the duty of the shareholders to pay the Zakah for their shares. If each shareholder could tell the Zakah due on their share, if the association paid Zakah in the form stated above, then he can pay it based on that. However, it is more deserving that poor members of the association aren`t given from the Zakah to avoid giving the poor from the Zakah due on their wealth.
As for not paying Zakah since the association`s establishment, the amount of Zakah should be calculated and paid to the eligible recipients to be cleared from liability before Allah. And Allah the Almighty Knows Best.