1. Open the "processors/join.php" and on about line 139 look for ASTAT_UNVERIFIED and change it to ASTAT_ACTIVE. This automatically active their account once they complete the signup process and they will immediately be able to login and access their account.
2. A confirmation email will still be sent out but you can edit and remove the activation and link can make it just a simple thank you for joining type email. Open the skins_site/def/emails/confirm_reg.html file and edit it to say whatever you want.
3. Once they complete the signup they are taken to an info page that tells the user that a confirmation email has been sent and they need to respond to this email before they can access and use their account. You can edit this and make it more of a thank you for joining message and that they can now login and access their account. Open the skins_site/def/info_signup.html file and edit it to say whatever you want.