Glad to hear you're interested!

The --url argument works exactly the same way as -u in Phoenix. In fact, it uses the exact same code, so anything usable with Phoenix works with that.
The admin-user and admin-pass arguments specify the administrator account on Multiminer, which can be used to log workers into the server and, since it is an administrator, hit Multiminer with JSON-RPC queries.

In short, you would use something like this:
multiminer --admin-user=admin --admin-pass=admin101 --web-port=80 --url=http://BitcoinUsername:BitcionPassword@localhost:8332/
EDIT: You can also specify a parameter like -b 30 and it will send 1/4 the work to each miner, which is a bit more efficient. You'd have to play with it a little though.
EDIT: As with Phoenix 1.4, the default askrate is 10, but you can change it with ;askrate=X, as in Phoenix.

And then you would connect Phoenix to Multiminer via MMP, with this URL: mmp://admin:admin101@localhost?name=SomeName

Of course, you are free to change the username and password for the admin account.

It's also worth noting that Multiminer is a complete webserver (the web-port=80 makes it run on the default HTTP port) that serves pages from the "www" directory, which means you can put a stats page in there to quickly see how your miners are doing, which is what jedi95 and I did.
You can also hit it with JSON-RPC requests. It's compatible with RPC miners (although it doesn't offer long-polling yet)