Speaking as someone who suffers from both conditions, captchas are not a significantly worse problem for depressed people than for others—they're impersonal, and while irritating, they set a fairly low bar for effort. Dealing with machines being machines is comparatively easy if you're able to make the effort to fill out the join-up form at all.

Asking someone for something, on the other hand, is high-effort for many depressed people for a couple of reasons:

1. It requires you to feel worthy of help, because if you're certain you're going to be refused, why bother trying? Depression and low self-worth tend to go hand in hand.

2. It requires you to risk refusal. Even if the other person's reason for refusing is neutral ("I no longer do that for anyone," for example), it can feed back into the depression and make it worse. Since this can hurt one hell of a lot, you learn not to ask.

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It's true that some people won't be able to scrape together enough interest or effort to pass even the captcha, but this alternative is much worse.

The issue with the group network version is that a few large corporations would end up taking it over. Again.