Note: You need to be an Owner or Admin in Alexis to be able to initiate an onboarding in AlexisHR.
As long as AlexisHR hasn't sent out the invitation email yet then you can always resume an onboarding setup if you want to. You can see who has received an invite or not yet through their status on the list of employees in Alexis.
How to
Go to People and find the person you want to resume an onboarding for.
From there go to Resume onboarding which is found in the Status column and proceed to make the edits you want or to complete the setup.