@dadagong - If you mean "Have the application open", then no.
If you mean "Do they need to have a license", then it depends on what you want the Flow to do and how they connect.
Sorry, I am new with MS Flow. Here is what I was trying to do:
1. create a flow from a template, I selected "Start approval when a new item is added" template and under This flow will connect to: Office 365 Outlook, i get invalid connection error.
2. I tried to create a flow to notify me when an new email arrives, when it prompted me to enter the email address, I got the REST API error.
Does this mean that I need to move my mailbox from prem to outlook 365?