Like Ezekiel, I don't find it completely useless because often you have options that achieve the result equally and you're reaching out for critique.

It's been mentioned earlier but Wake seems like it'd be the most popular solution (or competitor) for what you need.

Personally, I wouldn't find it very valuable. It'd add "one more app" to the stack and although I do see and appreciate the problem, using Slack (as you mentioned) or Invision prototypes representing mini flows (which is what I more often do) isn't inconvenient enough for me to sign up, incorporate into my workflow and get my team on board…

That said, don't let me (or anyone else) stop you if it's clawing at you! There's a lot to gain from building a side project (even if it does fail). It's good for experience and fantastic for job prospects — it shows you're passionate, capable and independent.

But if what you're after is less about the experience of building something and more about validation, like Paul mentioned earlier, build a landing page which concisely communicates what your app does, the problem it solves and why teams should care. Get their email addresses and reach out to them for concept validation, ideas and testing.

Whatever you choose, good luck!