Since 2009 the presiding economies started to invite several non-member countries and institutions to attend Leaders' summit. Until 2010, this worked on an ad hoc basis, guided by the following informal rules:
- the number of invited non-member countries will not exceed five;
- fair and balanced geographical representation;
- Spain will be a permanently invited country;
- countries presiding over regional forums , such as the African Union, ASEAN, APEC, etc., will be invited in the first place
The Seoul summit formalised the participation of five non-members, of which at least two would be African countries.
In addition to attending the leaders' meetings, the invitees are involved from day one alongside G20 members in the drafting of summit decisions. Invitees also take part in working group discussions, Sherpas and Finance Ministers meetings.
The G20 maintains close relationship with formal international organizations, such as the International Monetary Fund (IMF), the World Bank (WB), the Financial Stability Board, the Organization for Economic Cooperation and Development (OECD), the World Trade Organization (WTO), the United Nations and International Labor Organization (ILO) regularly take part in G20 meetings. Other international organizations are frequently also invited to participate in the premier forum.
Traditionally, international organizations provide expert support to the G20, where they take on various assignments and provide expert advice on matters within their competence. These organizations are actively involved in drafting reports, position papers and proposals on the primary agenda items.