Airtable has both free and premium plans available. The usage limits for each are described on the pricing page at airtable.com/pricing.
You can add as many collaborators and bases as you'd like for free, as long as each base falls below the per-base record limits described on the pricing page. If any of your bases exceeds those limits, you'll need to upgrade the workspace to which that base belongs.
For Pro and Plus paid workspaces, you will be charged the per-user, per-month fee for each person you've invited to collaborate on bases belonging to that workspace. You will not be charged for read-only collaborators on either of these two self-service plans.
If you'd like to upgrade to a paid premium account, please follow these instructions.
For Airtable Enterprise plans, billing is calculated differently. Please contact your sales representative if you have further questions.