How do we add another apple ID on an existing shared family plan?

See the following from Set up Family Sharing - Apple Support

Invite people to join your family

As long as you all use iOS 8 and later or OS X Yosemite and later, you can add anyone who has an Apple ID to your family. If you have children, use these steps to create their Apple ID and add them to your group. Use the steps below to add someone who already has an Apple ID.

On your iPhone, iPad, or iPod touch:

1. Go to Settings > [your name] > Family Sharing. If you're using
2. iOS 10.2 or earlier, go to Settings > iCloud > Family.
3. Tap Add Family Member.
4. Enter your family member's name or email address and follow the onscreen instructions.
5. If you’re using iOS 11 or later, choose whether you’d like to send an invitation via Messages or to invite them in person. Then follow the onscreen instructions.

On your Mac:

If you're using macOS Catalina:

1. Choose Apple menu  > System Preferences.
2. Click Family Sharing.
3. Click Add Family Member, then follow the onscreen instructions.

If you're using macOS Mojave or earlier:

1. Choose Apple menu  > System Preferences, then click iCloud.
2. Click Manage Family.
3. Click the add family member button (+), then follow the onscreen instructions.

If your family member is with you, they can enter their Apple ID password on your device to accept the invitation. You can also send them an invitation, and they can accept from their device. If you have multiple Apple IDs in the iTunes Store and the App Store, you can invite each of your accounts to the group, so you can share purchases from your other Apple IDs with your family.