Riba

It is not permissible to do anything related to usury (RIBA). The money received for such work will be also not lawful (Haraam).

Wassalam.

Increase in value does not create any problem if the activities of the company are lawful and no Haraam activities are involved like RIBA. If the company deals with RIBA or does any Haraam business, it will be Haraam to buy or keep such shares. Money generated from RIBA or any other Haraam activities is unlawful to use in any way even in charity. It must be given to the Marje; of Taqleed to spend it on deserving cases or seek his permission to give it in charity.

Wassalam..

If you know that it is a Riba between Muslims, then it will be the not permissible to deal with it. If the finance is given by a non Muslim to a Muslim, then it is permissible if it is done according to their own laws.

Wassalam.

Jurisprudence is a big field of Islamic knowledge which has its own experts who spend decades of years to be able to study the evidence from Quranic verses after going through all the fields of Quranic studies, and the authentic Hadeeths after going through the fields of Hadeeth and Rijaal which deals with assessing the authenticity of every narration. We also have the principles of Fiqh which is logic of jurisprudential thinking. 
'You need to go through the evidence confirmed by the great jurisprudence scholars or Maraaji' of Taqleed to see the reasons of any Islamic rule including your question.

'In brief, Islam does not permit RIBA (usury) between Muslims, but not every interest known today is usury. When non Muslims keep your money and put is their business and give you whatever they everyone else from their profit, it is permissible for you to accept it. This is according to the jurisprudence evidence which has details in jurisprudence books written by the Maraaje' of Taqleed. You are accepting what they usually give to every one who keeps his money with them as part of their business.

'Wassalam.

The main point here is the activities of the company. If the company deals with unlawful activities like usury (RIBA), alcohol, gambling, cheating etc, then it is not permissible to work in such company regardless who are the owners or clients. 
'It is also important that the capital of the company should not be earned through unlawful business.
'Wassalam.

It is not permissible for Muslim to work in Riba and the income from such unlawful industry is also unlawful.

Wassalam.

If you are sure that you will never help in usury or any unlawful (Haraam) work, then you can work in such legal firm.

'Wassalam.

If you mean that the company rewards you with more tokens if you do more business with them or vote for them or bring more customers, then this reward is allowed
for you. This is when the activities of the company is investing in lawful business only.

Wassalam.

If the amount is been invested in lawful business away from unlawful business e.g. alcohol, pork, usury etc, then the income from it will be lawful. Such income can not be fixed by a percentage from the capital, but it must depend on the profit if any.

Fixed amounts given by non Muslims is allowed for the Muslim to receive and utilize in lawful needs and aims.

Wassalam.

We can not prohibit others in Western society from dealing with Riba, but we as Muslims are not allowed to deal with Riba.

The meaning of Riba is to be taken from the Marja' of Taqleed and not from the people.

Despite the fact that we live in a society which deals with Riba but we should not indulge ourselves in Riba as far as we can.

Wassalam.