No, it is a requirement that each person use an unique email address when joining Geni. If the couple shares an email address, you may wish to request that they consider signing up for another free email account; otherwise, you may wish to only invite one of them to join your tree.
Please note that one's email address not only becomes the user's login name- it also becomes a way for the user to verify their password, receive Family Newsletters, etc.
Note: If you have a gmail account, you may simply add a "+" sign to create multiple email addresses. For instance, if your email address is johnsmith [at] gmail [dot] com - you can use johnsmith+mary [at] gmail [dot] com as a second email address for your spouse, etc.