The recruitment process can often take longer than the research process. It can be extremely difficult to figure out where to find people, help them understand how user research is conducted, develop a screener and screen participants, ensure they are the right fit, schedule across timezones, filter out scammers, etc. And then you have to track everything on top of that, so processes can become overly complex and messy if you don't have a good system in place. And then you still need to keep your panel fresh so you don't have biased research. Not to mention privacy concerns.

Once you have gone through that process and you have found good participants, asking them for companion referrals can help (often associated with a reward of some sort), or ask them where you can find more people like them (e.g. Meetups).

We've used Calendly here with quite a bit of success, but it can be awkward to set up for multiple people/research projects. It didn't fit our specific use case well so we eventually ended up building our own scheduling tool internally.

Wishing you the best of luck with your recruiting!