You can claim your adult child as a dependent if the individual lived with you the entire year, made less than $4,300 in 2020, and you provided over half their support.
Multiple people each of which provide at least 10% of the support of an individual can complete a Form 2120 Multiple Support Agreement and decide which will claim the dependent on their tax return.
The dependent can still file their own tax return to claim a refund of withheld tax and indicate they are the dependent of another taxpayer on their tax return,
