Upgrade issue on Zoho Docs

Hello,

I am the admin of my organisation. I have upgraded myself to a 100GB plan a few days ago to be able to share more things with the 3 more users within my organisation.
Today I tried to add one more user and it says I have reach my user's limit... And I realise that the 3 users I had already added have an inactive status.
I don't understand...
If the admin pays to upgrade to a 100GB plan it means that all users withing the organisation have to pay? They cannot keep their free plan and access my documents?
I missed something doing this upgrade... It is really not clear.

Please advise me,
Many thanks