A Sukkah with joint ownership:[1]
One fulfills his obligation with a jointly owned Sukkah even if he enters it without the permission of his partner, [and he is thus not required to request permission from his partner in order to dwell in it].[2]
[1] Admur 637:3; Michaber 637:1; Tosafos Sukkah ibid; Rivash 347
[2] The reason: This is not considered stealing as it was under this premises that the partnership was made, that each partner be able to use it as he wishes. [Admur ibid; Rivash ibid]