Agorapulse plans have switched to per user pricing. Each plan is purchased for 1 user and each additional user on the plan will cost the same as the first user. This is explained in more detail here.
Free, Standard, Professional, Advanced plans: single user
Custom plan: customised
Free trial: you can invite team members for free during the Trial period.
At the end of your Trial, you can either go to a Free plan or Subscribe. When you subscribe this is when you will have to pay for those previously invited users. 1 invited user = 1 user seat
If you want to purchase a lower number of seats, you'll need to remove any active users or pending invitations you don't wish to pay for.
In general, if you need to add an extra user(s), you can do this by going to your Profile Picture icon and then click Subscription:
There you can click the Edit plan button to add another user or more social profiles. The price for each additional user will depend on the plan you are subscribed to - pricing information is detailed here.