Should they ask for their salaries to be increased like the salaries of their colleagues who are sponsored by the Association?
If the matter is as described, and you do the exact same job as your colleagues, then it is your right to ask for the same salary as them, exactly the same regardless of whatever you are paid for other work, because the employment contract is about working a certain number of hours in return for certain wages, and outside of these hours he may do whatever work he wants in return for payment or whatever, so long as that will not affect his work in his regular job.
Treating employees fairly and reducing differences in their salaries is the strong foundation for achieving success, spreading tranquillity and removing resentment and grudges, especially if this is in da’wah work which is undertaken by those who are expected to be good, righteous and adhere to the limits of sharee’ah.
To sum up, you have the right to ask for your salaries to be raised and to get the same time off as your colleagues at work are getting. The association and those who are in charge of it should pay attention to attaining fairness and reducing differences as much as possible, because there is goodness, prosperity and success in that.
We ask Allaah to bless and guide us and you.
And Allaah knows best.