As far as the number of joint authors is concerned, this one is hard to beat: Malmsten, C.J., Almgren, T.A., Camitz, G., Danelius, D., Moder, D.H., Selander, E., Grenander, J.M.A., Themptander, S., Trozelli, L.M., Föräldrar, Ä., Ossbahr, G.E., Föräldrar, D.H., Ossbahr, C.O., Lindhagen, C.A., Moder, D.H., Syskon, Ä., Lemke, O.V., Fries, C., Laurenius, L., Leijer, E., Gyllenberg, G., Morfader, M.V., Linderoth, A.: Specimen analyticum, theoremata quædam nova de integralibus definitis, summatione serierum earumque in alias series transformatione exhibens (Eng. trans.: "Some new theorems about the definite integral, summation of the series and their transformation into other series") [Dissertation, in 8 parts]. Upsaliæ, excudebant Regiæ academiæ typographi. Uppsala, Sweden (April—June 1842)
Edit (December 20, 2016): t turns out the information in Blagouchine's citation is imprecise and the number of Malmsten's co-authors has to be reduced. More detailed information which I received from Lennart Börjeson (thanks!) indicates first of all that Moder, Föräldrar, Syskon and Morfader are not last names but rather parts of respective dedications (translation: mother, parents, siblings and maternal grandfather). Indeed, in the scanned copy of the paper available here, goo.gl/bZIWZZ one such dedication can be seen on p. 2. Another point: the remaining names are those of students and the parts of the paper are probably their theses. This makes the co-authorship questionable, since before 1852 in Sweden it was common that the professor wrote the thesis bearing the student's name rather than the student himself. Still, the names appear jointly in print (only some of them can be seen here, but the paper version from which the scan was made might have been a part of a bigger series of publications).