Hi there and welcome
You will have to reset the password
To do this
Power off the Mac
Right after you hear the chime, press and hold the Command and the R keys for a few seconds, then release. Soon, you'll see the Apple logo and spinning gear (or progress bar).
This will force the Mac to boot into OS X on the Recovery Partition that was created when you installed
When booting is complete, you'll see a window of "OS X Utilities." You won't do anything there. Instead, you'll go to the Menu bar at the top and select Utilities > Terminal.
In the terminal window type the following and hit Return.
A new window will open, like this:
Click on the the main volume, then select the desired user account name in the popup. Then enter the new password (twice). A password ***** is optional. Click "Save."
Go back to the Apple Menu at the top. Select OS X Utilities > Quit OS X Utilities. That will prompt you to restart the Mac.
When the Mac restarts normally, you'll be able to log in to the administrator account with the new password ***** defined above. From there you can manage the rest of of the users if necessary with System Preferences > Users & Groups.
Let me know the results please