I mailed multi accounts @ this e-mail multi-accounts@swcombine.com

and it bounced back as undeliveriable.
I am trying to get my brothers account added to mine, so when he's on vaction he can log in to check his trades. BTW I copy and pasted the url from the contact us and that is the one that bounced back.

Can I DM someone from multis about this?

That address looks correct. No clue why you don't come through.

And the one in charge for multis is Zephlin Abath.

First, I'd try it again because the mail server may just have been having issues. If that fails, then drop Abath a DM for your request, and just tell him you had problems with the e-mail address.