Invitations
From StatusNet
Inviting a friend to identi.ca or any other StatusNet based platform is pretty easy. Using the web interface you find an Invite link in the main menu. Alternatively you can directly access it at SERVER/main/invite; so for identi.ca it is http://identi.ca/main/invite.
Following that link, you'll reach a form with two boxes and a send button. In the upper box type as many email addresses as you wish to invite (one per line) and leave a personal message in the lower box. Then hit the Send button.
Invitation emails consist of a short introduction of the server and who invited you. With links to the profile of the member who send out the invitation and to register at the server.
If the invited accepts the invitation you'll be informed. If s/he is already at the server you'll be automatically subscribed to that account.
[edit] Public Registration
Public registration is available at SERVER/main/register but is unavailable on Private servers (the default mode). On private servers, users must be invited.
[edit] How to find before Laconica 0.7.3
StatusNet was previously known as Laconica.
If the main menu is missing the Invite chances are high, that the platform you are at is not running the latest version of Laconica. In the early 0.7 versions that link was pretty hidden.
You can find it in the web interface from your profile following the All subscriptions or All subscribers link in the sidebar. Directly accessing the page is unaffected from the interface changes.