It depends on multiple factors:
- Client / Employer relationship contract
Like others have mentioned, not all clients are willing to give up or do the effort to help you recruit and reach their users.
Some clients understand the value of talking to users and will gladly give you access to their user base with them as the intermediary or not. However you will have to make clear that you, as the researcher, have complete control over who to pick, how to contact them, the exact languages used etc. Most clients don’t realize research follows a set of deliberate processes that cannot be tampered with.
- Objective of study
If you are optimizing an app for existing users, access to existing users will help achieve that goal. But carefully discriminating who to test is critical for your findings. Have you noticed a set of users performing differently and want to explore them further? Are you optimizing for expert users? Etc.
If your objetive is bringing in new users, then you are targeting current non-users. In this case, the client won’t be able to help. It will be up to you to reach them (craigslist, meetups, placed were you find them, online forums, recruiting agency, etc).
- Nature of product
Some products are B2C, others B2B. Consumer products are normally easier to find users for since there are plenty and easily accessible. Business products require professional users under specific industries, who will be especially hard to reach and would require a considerable higher compensation for their time.
- Cost for recruiting & Accessibility
Will the client pay for compensation? Will your company pay? Who are you testing and what is a correct compensation for their time? Doctors could demand $100+ per session while a consumer might be do well with a Starbucks Giftcard.
How will you access professional users? Your client will most likely need to provide them.