How to send bitcoin to a multisig address I am trying to deposit bitcoin to bitmex, which only offers multisig wallets.

I have tried to send from both GDAX and Jaxx, but the addresses are reported as invalid. 

Apologies for the basic question, but how do I send to a multi-sig address?
You send bitcoin to a multisig address in the same way as a normal address. Multisig addresses are just a type of P2SH address which start with a 3, and have been supported by every major wallet and exchange for years.

If your address doesn't start with a 3, it's probably not an address at all. (Unless it starts with bc1 which is a very new address format and not widely supported yet)