There is an easier way to do this.
I have Google Inbox so automaticly any account that is in my phone gets Inbox.
What I will do is as follows:
I will create a burner gmail account, link it to my Inbox and then unlink it, I will then share the login with others, you will add the burner gmail account to your phone, then you will go into the Inbox app with it, once you are there just select your main gmail account, and viola you have Inobx.
I will share the login with the first person who PM's me, and he should in turn pass it on to the next person and so on. Google may block access to the account being that they suspect fraud, so it would be best to also include your login city when you share the login info with the next chap.