Mainsite and meta aren't set up for this.
We don't allow "Who wants to join my group?" questions for a whole host of reasons, not least of which is that answers will be "Yes, I want to!" and "No, I don't." Not exactly insightful experience-based solutions to a problem you're facing--the answers are useful to YOU but not to anyone else in similar situations who aren't you.
We want the questions on our site to be valuable sources of information for the whole Internet, not just our community members. (from What Topics Can I Ask About Here)
Playtesting solicitation is just a variant on "join my group" and has all the same problems.
Chat can do it, but not well.
You're free to solicit for playtesters in chat, but you may not get a big response: it's a small, erratic group from all around the world. A joining of interest and availability on a scale large enough to do playtesting is uncommon, since good playtesting needs a fairly wide variety of players and situations in order to usefully evaluate the subject.
You can ask the main site for guidance in finding players, though!
This would actually make a good mainsite question: "I have X system/adventure/whatever, with Y qualities, that I want to test for Z audience. How do I get playtesters?" The Stack is set up to provide this kind of assistance. It can be answered through experience; it has clear criteria for voting on whether answers are useful; its advice can be useful to others in similar but not identical situations.