If you wish to invite users who already exist in another account, you can do so using the Merge Accounts feature.
Step 1: Log in as an administrator to your account and go to User Admin.
Step 2: Select Merge Accounts
Step 3: This will prompt a new dialogue box to appear. Add the name of the administrator of the account that you wish to invite into your account.
This will send an email to the administrator of the other account inviting them to merge themselves and all other users in their account into your account.
They should simply follow the instructions in the email and they will merge into your account.