Invite code generation
If you want to leave invite codes enabled on your installation, you'll need to know how to generate them for users! There are a few ways you can do it. All of them require the user who is generating the invite codes to have been granted the 'payments' priv, and for that user to be logged in while granting the invite codes.
A user who has been granted the 'payments' priv visits the Admin Console while logged in (/admin/console) and generates one or more codes for a single user. The command to generate invites is:
make_invites username number "reason"
make_invites system 5 "to create dev accounts"
The above command issues 5 invite codes to the 'system' user, with a note (where?) saying "to create dev accounts".
Multiple user grants
To give out invites to all users (or all users who match certain criteria), a user who has been granted the 'payments' priv visits the invite codes distribution page at /admin/invites/distribute and fills out the form there. There are multiple criteria for distributing invites.
This is a two-part process.
Recipient: The user who needs the invite codes (either you logged in as that account, or the account owner, if that is a different person) visit the Invite Someone page at /manage/circle/invite and click the link to request more invites.
Sender: Make sure that the user who will be sending the invite codes has been granted the 'payments' priv, and visit the invite codes requests page at /admin/invites/requests while logged in as that user. You'll see the request there and can choose how many invite codes to grant.