Hi there donaldfromny,
If the family members are already part of your Family Sharing group, you don't need to send any additional invitations.
When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.
If they have their own iCloud+ storage plans already, they can switch to the shared plan with the steps here:
If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.
On your iPhone, iPad, or iPod touch
In iOS 16 or iPadOS 16 later:
1. Go to Settings > Family.
2. Tap Subscriptions.
3. Tap iCloud+.
4. Tap Use Family Storage.
In iOS 15 or iPadOS 15 or earlier:
1. Go to Settings > [your name].
2. Tap Family Sharing, then tap iCloud+.
3. Tap Use Family Storage.
You'll find those steps and more information here: Share iCloud+ with your family - Apple Support
Take care.