How to enable guest posting in phpBB3
The original forum permissions in phpBB3 give guests read-only access. This means that if you copied the original permissions from the example forum that comes with the installation package to your new forums, guests can't reply to topics but only read them. Of course, you can change the permissions of any forum on your board and give guests more or less rights.
When it comes to spam protection, it's better to have guest posting disabled, but the choice is yours. You may prefer to give guests the permission to post messages.
To enable guest posting, log in to the administration control panel of your phpBB3 board, and either click on the Forums tab or on the Permissions tab. Then click on the Forum permissions link on the left and you'll see a list of all your categories and forums. Select all (or some) of the forums and click on the Submit button. On the next page select Guests from the Manage groups menu and click on Edit permissions.
On the next page (Setting permissions) there is a drop-down menu labeled Role. With the default permissions it's set to Read Only Access. You can click on the Advanced Permissions link to see in details exactly what guests are allowed to do with this role. You can use the Role drop-down menu to select one of the preconfigured roles. For example, Limited Access gives guests the right to post in forums. Alternatively, you can use the advanced permissions to allow guests to post messages, and anything else you want them to be able to do. Then click on the Apply permissions button to save the changes.
From the same options you can also deny guests access to any of your forums (including read access).
Make sure that if you enable guest posting, CAPTCHA (visual confirmation) for guest posting is enabled. Otherwise, spam bots will make your life miserable. CAPTCHA is enabled by default. To check go to the General tab of the admin panel, then click on Spambot countermeasures on the left, and make sure that Enable spambot countermeasures for guest postings is set to Enabled.