There is no such thing as a joint IRA, IRAs are owned by individuals only. However, a non-working spouse may contribute to an IRA using their working spouse's wages as the basis for the contribution.
To contribute to a Roth IRA, you must have compensation from working (wages or self-employment income). Your contribution limit is $7000, or $8000 if you are over age 50, or your compensation, whichever is less. Your spouse can contribute from your compensation ($7000 or $8000) a up to the amount of your compensation minus your contribution. (For example, if you are both over age 50 but your only wages were $10,000, you could contribute $8000 to an IRA in your name but your spouse could only contribute $2000 to an IRA in her name).
Then, if your income is over a certain limit, you may be completely disallowed from making any Roth IRA contributions, that is here.
https://www.irs.gov/retirement-plans/plan-participant-employee/amount-of-roth-ira-contributions-that...
If you can't make a Roth contribution because your income is too high, you can still make a non-deductible contribution to a traditional IRA. This has certain additional complications, especially if you already own an IRA, but we can go into that if needed.
Participation in a qualified workplace plan (401k, 403b or other qualified plan) does not affect your IRA limits. Even though the plans have a similar purpose, they have different limits and are covered by different laws, so a 401k does not disqualify you from contributing to a Roth IRA as long as your income is otherwise eligible.
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