Unlike IRA’s, 401(k)’s, and other employer sponsored plans, joint brokerage accounts do not have contribution limits. Individuals can contribute as much as they would like to a joint account. At Betterment, any individual above the age of 18 can contribute to a joint account, regardless of their AGI (Adjusted Gross Income).
While joint accounts may not have contribution and income limits, it's good to keep in mind that these accounts are not tax sheltered and may be subject to capital gains tax. Read more about how taxes may affect the investments in your joint account.