Multi-User Chat (XEP-0045)  defines a protocol for groupchat over XMPP. That specification includes a method for inviting a contact to a room, where the invitation is mediated by the room itself: the user sends the invitation to the room, which in turn sends it to the contact. Unfortunately, a mediated invitation might not be delivered to the contact, for example if the contact blocks communication with entities not in its roster as specified in Privacy Lists (XEP-0016) . As privacy lists have become more common, MUC invitations have been increasingly blocked at the server side, resulting in an undesirable user experience. Therefore, this specification defines a method for sending an invitation directly from the user to the contact, which re-uses the original 'jabber:x:conference' namespace in use before XEP-0045 was written (with the addition of 'reason', 'continue', and 'thread' attributes for feature parity with mediated invitations).
A user invites a contact to a room by sending a <message/> stanza containing only an <x/> element qualified by the 'jabber:x:conference' namespace.
The 'jid' attribute, which is REQUIRED, specifies the address of the groupchat room to be joined.
The 'password' attribute, which is OPTIONAL, specifies a password needed for entry into a password-protected room and maps to the <password/> element in a mediated invitation.
The 'reason' attribute, which is OPTIONAL, specifies a human-readable purpose for the invitation and maps to the <reason/> element in a mediated invitation.
The 'continue' and 'thread' attributes, which are OPTIONAL, specify that the groupchat room continues a one-to-one chat having the designated thread; these map to the <continue/> element (with 'thread' attribute) in a mediated invitation. Note that the 'continue' attribute has a datatype of boolean. 
Upon receiving a direct invitation, the contact's client SHOULD present the invitation to the contact so that the contact can accept it or decline it. If the contact accepts the invitation, the contact's client shall join the room as described in XEP-0045. If the contact declines the invitation, it shall silently discard the invitation.
If an entity supports the protocol specified herein, it MUST advertise that fact by returning a feature of "jabber:x:conference" in response to Service Discovery (XEP-0030)  information requests (see Protocol Namespaces regarding issuance of one or more permanent namespaces).
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) . However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
Before inviting a contact to a members-only room, a user SHOULD check to see if the contact is already a member and, if not, add the contact to the member list (or ask a room administrator to do so).
If a client receives multiple invitations to the same room (e.g., a mediated invitation as defined in XEP-0045 and a direct invitation as defined here), the client SHOULD present only one of the invitations to a human user. If a client receives an invitation to a room in which the user is already an occupant, the client SHOULD silently discard the invitation.
The following attacks are possible, in roughly the order of probability. See also Best Practices to Discourage Denial of Service Attacks (XEP-0205)  and RFC 3552 .
The sender of an invitation could overload the 'reason' attribute with malicious or offensive text. The recipient can mitigate this attack by blocking the sender using technologies such as Privacy Lists (XEP-0016)  and Blocking Command (XEP-0191) .
A passive attacker could flood the recipient with a large number of chatroom invitations. This attack, too, can be mitigated with Privacy Lists or Simple Communications Blocking.
A passive attacker could use a mimicked JID to fool the recipient into thinking that the sender is a known or trusted contact. This attack requires knowledge of the recipient's known or trusted contacts, and can be mitigated by following the recommendations in Best Practices to Prevent JID Mimicking (XEP-0165) .
In the absence of end-to-end encryption, a passive attacker could eavesdrop on the chatroom invitations that a user sends or receives. This is especially threatening if the invitation includes a 'password' attribute for a password-protected room.
In the absence of end-to-end encryption or signing, an active attacker could modify the invitation in transit so that the recipient is directed to a different room than intended by the sender.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar  includes 'jabber:x:conference' in its registry of protocol namespaces at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
Thanks to Joe Hildebrand for his feedback.
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3. In accordance with Section 184.108.40.206 of XML Schema Part 2: Datatypes, the allowable lexical representations for the xs:boolean datatype are the strings "0" and "false" for the concept 'false' and the strings "1" and "true" for the concept 'true'; implementations MUST support both styles of lexical representation.
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Added continue and thread attributes for feature parity with mediated invitations when converting a one-to-one chat to a groupchat.
Added password attribute for feature parity with mediated invitations.
Per a vote of the XMPP Council, advanced specification from Experimental to Draft.
Defined several possible security attacks along with solutions.
Added optional reason attribute for feature parity with XEP-0045.
Initial published version.
Modified to use old jabber:x:conference namespace.
Added note about handling multiple invitations.
Added implementation note about members-only rooms.
Changed venue element to conference element.