Registering/adding users with an apostrophe in their email address ([email protected])
C5 checks emails for validity using a list of allowed characters. the apostrophe is not one of them. My guess is that although apostrophes are technically allowed in emails, their support is limited with some email providers rejecting them.
If you really want to allow the use of emails than you need to create a modified version of the file ROOT/concrete/core/helpers/validation/string.php
Modify the regex in the function email allow apostrophes.