It's not currently possible to have multiple members with the same address. We use the e-mail address as the unique ID for each member.
Members with no e-mail address
You can use a fake e-mail address such as firstname.lastname@example.org to store records for members with no e-mail addresses. To avoid their record being flagged as invalid you need to opt them out of e-mails by editing the record from within your member database and clicking on the blue 'opt-out' button. However, if you're storing additional information on these members in custom fields in your database, then the record will never get deleted.
Multiple members sharing one address
Some groups might need to have multiple member records (for a family with children for example) but only one e-mail address. It's not possible to use the same address for different records, but you could either use a fake address like above for the additional members, or add "+1", "+2" and so on before the @ sign of the primary members' address for each additional member record. In Gmail, this will only send one e-mail to the primary member, but in some other e-mail systems and clients, an additional, duplicate e-mail will be sent for each additional member, so a fake address might be preferable.