Gmail’s new Inbox app is pretty awesome, but it’s still invite-only.
Have your friend log into their Google Account to your phone.
For Android, you’ll want to do this under parameters > Accounts.
On iOS devices, enter through another Google app, like Gmail or Chrome.
Open Inbox and, when prompted, choose their account to sign in.
Go to Inbox’s configs and select “Manage Accounts”.
Flip the switch for your account to “on”.
You should now have access to Inbox on your account.
Remove your friend’s account from your phone.
Again, on Android, do this through controls > Accounts.
According to WonderHowTo, this only works for one degree of separation.
Give Your Friends Access to “Inbox by Gmail” Without Any Invites| WonderHowTo