You can add as many members to your Teamspace as you'd like. If you are on a paid plan, you will be charged for an additional seat for each member who accepts the invitation.
How billing works:
You're only charged when someone actually joins, not when you send the invitation.
Each new member adds one seat to your current plan at the same rate.
Billing is prorated - you pay for the remaining days in your current billing cycle.
Example: If you're on a $30/month plan and add a member halfway through your billing cycle, you'll be charged $15 for that member for the remaining 15 days, then the full $30/month going forward.