I want to add my husband to my existing family storage plan how do I do that

Read this link:

Set up Family Sharing - https://support.apple.com/HT201088 -

"Invite people to join your family

"Remember, you can only join one family at a time, and you can only switch to a different family group once per year."

After you send the invitation, you can check its status under the person's name in Settings > iCloud > Family on your iOS device. On your Mac, go to Apple menu > System Preferences, click iCloud, and select Manage Family. Then select that person's name to see the status of the invitation.

If you need to resend the invitation, select Resend Invitation."