You can invite anyone that either does not have a mobile plan or is with another carrier! Like you, their device must be unlocked. When they sign up they need to be the primary line and create a brand new Reach account.
You'll get referral credit for any new Reach subscriber who opens an account. New line add-ons don't qualify for referral credit.
You can qualify for one free month of service at a time, up to a $75 value each month. Any subsequent referral credits you receive will be applied to the following month. For example, if you have 3 referrals activate in one month, you would get the following 3 months of service free (valid up to $75 off your bill each month).