You can add or invite employees to your CORE company only if you are the Owner of the account or Administrator. You can do so from the Manage Users screen under your company menu > Account. Make sure that you properly add the employees in Account > Manage Users. When they have been given a subscription, they should receive invitations as normal. If this doesn't work, try re-sending the invitations to the users. Check CORE Help Center for details. However, if the users still do not receive the email, please contact BQE Support.
CORE also allows you to add vendors (outside consultants or contractors) as users. This lets them track their time on the projects worked for you. When you send an invitation, they can accept that and sign up for their account. Check CORE Help Center for details.
If you want to resend the invitation for CORE subscription to a different email, you can do so by deleting the user's account first. Then update the email and add the user again, resending the invitation to the new email address this time.