I think you're sort of missing the point of Eris by asking of "official" ways of summoning Her. There is an Eris Invocation on page 50 of the Apocrypha Discordia (this is a direct download link), but it (like most rituals in the A.D.) requires 5 participants. If you are looking for generally Discordian rituals, the Apocrypha Discordia is a great place to look. My advice would be to just make something up; that'll work better than any sort of "official summoning spell". Eris isn't into tradition & the official.