Joining the Royal Arch (or 'Chapter') is the fourth and final stage in Pure Antient Masonry, as defined in the Act of Union of 1813 that created the current United Grand Lodge of England.

Anyone who has been a Master Mason for four weeks can join—and should aspire to join the Royal Arch, because becoming a Companion of the Order continues their voyage of discovery.
Source: www.ugle.org.uk/about-us/royal-arch