customizable invite-new-user text

[brainwane]

It would be great if the "Invite users to Zulip" dialog also gave me a way to add optional personal/custom text that would be included in the invite email. For instance, I'd like to be able to say, "I'm inviting you to this Zulip realm because we're having a chat about trains in train on Monday at 3pm."

[timabbott]

Yeah, this would be a nice improvement

[jainayush975]

I am starting to work on it.

[brainwane]

Great, thanks, @jainayush975 -- looking forward to your pull request!