How do i create a rails app so admin creates user accounts

Hello there I am new at rails and I got kinda a suggestion/request to make an app similar to slack, basecamp etc.. For the company ( where i am an intern). So I got no idea how to make the part where the admin creates the user account and then invites or just gives the account to the user and then he changes the password of his account. I figured out the three main tables that I am going to work (User,Post, Event).
How would you suggest me do the admin part. Thank you. <3

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