I’ve seen this somewhere before and it worked really nicely. But I can’t remember where.
Anyway, here’s my desired form.
Enter a message:
+--------------------------------------------+
| |
| |
+--------------------------------------------+
|
| Enter recipients:
| +------------------+
[Send to everyone] OR | |
| +------------------+
|
| [Send to these users]
You mean like on StackOverflow?
Just inspecting the DOM will tell you what you need to know: The line is an absolutely-positioned element with a left-border, and the “or” has a background colour the same as the body to cover up the line around it.