Will the Liquid sidechain be open to any user?
Liquid wallets are open and accessible to anyone. This could either be SPV wallets such as Blockstream Green or the Elements Liquid node. 

With these wallets you can verify transactions (your own transactions with Green, all transactions with the node). However, you can't produce blocks. To do that, you need to be one of the Liquid members. To become such a member, you need to go through Blockstream. At the time of writing, there are 33 members full list available here.