The rule is simple: every domain gets its own row under Connections, Your Sites, and its own line of code. That is what gives each site its own name and colour on the globe, and it is also your security: your site IDs only work from the domains on that list.
Another WordPress site Best: install Pulse there and paste the pairing code. It needs no licence key, and logged-in members are recognised by themselves. Quick alternative: paste that site's one line of code in the theme footer.
A plain HTML page Paste that site's one line before </body>. Done. It works when you open the file from your own computer too.
A PHP site or script Paste the same line into the footer file every page includes (often footer.php), once.
A membership site Put the line in the members template as well as the public one. Pages behind a login count exactly like public ones, because the line runs in the member's own browser.
A sub-domain members.your-site.com is covered when your-site.com is on your list. Give it its own row only if you want it in its own colour.
Showing who a logged-in member is (sites that are not WordPress)
On a WordPress site Pulse does this by itself. Anywhere else, your member area already knows who is logged in, so tell Pulse with one line after the tracker:
<script>window.addEventListener('load',function(){ RAVG.identify('member@your-site.com','Maria'); });</script>
Print the member's real email and first name there from your membership script. The email is turned into a one-way fingerprint inside their browser and only the fingerprint is sent. If the family knows that person, they show in your Live View by name with their lifetime value.
Any opt-in form on a tracked page does the same thing by itself the moment somebody submits it.
Forgot to list a domain? Nothing breaks quietly. Your Dashboard and the Connections tab show "visits from a domain that is not on your list" with a This Is Mine, Add It button. One click and that domain counts from then on.