To add additional users, you need to go to Pricing and find the window Add-ons at the bottom of the page. It is possible to connect additional users using any subscription, except Free Trial.

Are adding users deleted when switching to another plan?

Because another plan initially implies a different tariff and the addition of additional users is a one-time payment when changing a subscription, free places for additional users are deleted. 

If the user has been added to the team, will it be deleted? 

No, places for users are deleted only if no team at the place. If the additional users are already a member of the team, when changing the subscription, s/he will not be removed from the team. 

But if you delete this user, then his extra space will also be automatically deleted.