Occassionally an open source or hosted client will ask why if contact forms add a user record and profile that the same is not true for even attendees. In fact events is the only module that does not automatically try to match up a user record or create a new record. There is a reason for this. (Note: at the bottom of this help file is a work around that can be used now. But on to the isssue at hand.)
Scenario 1 - why we don't auto-add registrants as users:
AE for a sponsor buys a table for 10 - but doesn't know who is going to attend. They typically enter something like this:
Guest 1 email@example.com
Guest 2 firstname.lastname@example.org
Guest 3 email@example.com
Guest 4 firstname.lastname@example.org
Guest 5 email@example.com
Guest 6 firstname.lastname@example.org
Guest 7 email@example.com
Guest 8 firstname.lastname@example.org
Guest 9 email@example.com
Guest 10 firstname.lastname@example.org
Now you have 10 junk profiles and can't actually add a real person to that slot, they don't get reminders, etc.... Not cool.
Scenario 2 - why we don't auto-add registrants as users:
Even legit users don't login to register for an event. Not all companies support people joining organizations because they are afaid of head hunters. So lots of people will use personal email addresses for events so they match their personal credit cards. But, they want their main record to be associated with their company email because they are granted membership through a corporate membership. Again this creates a duplicate user.
Export event attendees and import them as users (this is an option on the event's roster report and in several other places. Imports are similarly documented in too many places to link.
If you really want attendees added to the database of users automatically it can be programmed. There are nuances like avoiding email addresses of superusers who tend to be the ones adding other users as even attendees. But it would be a great pull request from a github issue https://github.com/tendenci/tendenci/issues or even just starting a conversation on the forums at https://www.tendenci.com/forums/