We do not believe that one joint owner of an account could add an additional joint owner without the consent of all joint owners. The Joint Tenancy Act requires that all account owners sign the account agreement to create a joint account. 765 ILCS 1005/2(a)Bray v. Illinois Nat’l Bank, 37 Ill.App.3d 286, 288–89 (4th Dist. 1976).

However, because the account you described is owned in joint tenancy, both account owners have the right to withdraw 100% of the account funds at any time. Thus, we believe that the grandmother could withdraw her share of the funds of the account and open a new account with her desired co-owner. 765 ILCS 1005/2(a):

When a deposit in any bank or trust company transacting business in this State has been made or shall hereafter be made in the names of 2 or more persons payable to them when the account is opened or thereafter, the deposit or any part thereof or any interest or dividend thereon may be paid to any one of those persons whether the other or others be living or not, and when an agreement permitting such payment is signed by all those persons at the time the account is opened or thereafter the receipt or acquittance of the person so paid shall be valid and sufficient discharge from all parties to the bank for any payments so made.

However, she would not be able to withdraw more than her 50% share of the funds without accounting to the other joint owner. 765 ILCS 1005/4a.