All-Party Parliamentary Group

According to the UK parliament's website;

All Party Parliamentary Groups 'are regarded as relatively informal compared with other cross-party bodies such as select committees of the House. The membership of all-party groups mainly comprises backbench Members of the House of Commons and Lords but may also include ministers and non-parliamentarians. Groups flourish and wane according to the interests and enthusiasm of Members.Groups are classified either as subject groups (relating to a particular topic eg. forestry) or country groups (relating to a particular country or region)'.

Rules for all party groups can be found here.