This looks pretty good.

• It pings the first 5 valid users in the edit summary. If you put in 6 users and user #2 has a typo then user 1, 3, 4, 5 and 6 will get pings.
• I'm feeling a little uneasy about pipe support. Mainly because you cannot see what the wikitext was, nor can you fix it. See this example or this one.
• Works for anons, as expected.
• Mentioning people multiple times sends them only one notification, as expected too.
• People cannot ping themselves, also expected.

I seem to have found a bug. I have three users - [[User:Niharika29]], [[User:Niharika]] and [[User:Niharika2]].

1. Log in to [[User:Niharika2]] on ruwiki beta.
2. Open a random article.
3. Edit article using VE.
4. Add comment summary - Ping [[User:Niharika]] is a JERK!! And [[User:Niharika29]] is awesome!!. See example.
5. Go to both of those accounts on enwiki beta. [[User:Niharika]] hasn't received a cross-wiki notification but [[User:Niharika29]] has.

I have a guess at what might be going wrong. It's possible that [[User:Niharika]] account was not attached to ruwiki beta and that was the cause. Once I logged in to that wiki using that account, I started getting cross-wiki notifications. If that's the bug, then it's with Echo.

That's simultaneously a defect and not a defect — hypothetically the user should be able to be pinged (given global accounts) but technically the user account is technically not created at that wiki until the first pageview is made. I strongly believe we should leave it like this, both because this is a Echo issue and will take a lot of effort to dig into and address properly (planning, communication, and dev) but also because if a user has never viewed a wiki they should not receive correspondence from that wiki. They can be contacted at their home wiki or Meta.

😉