Hey ​@gintare , I confirm that on all paid plans, each Dev Mode seat will be billed. You’ll need to pay for their seat in order to grant them editing rights in Dev Mode.

To give someone access to Dev Mode, they’ll need a 'full' Design role on your team. You can only access Dev Mode in the file of the purchased team.
 
Just a quick reminder: each team is responsible for payment for all editors who have access to the team or your team's files, regardless of whether they belongs to another paid team or not. This is since team billing happens independently from the other.

(For additional context, Starter users, or viewers on paid plans won’t be able to access Dev Mode's advanced features in files, but they can still do basic inspection like viewing properties and measurements, copying code, and exporting asset.

Developers who need full access to Dev Mode will need an upgraded seat on a paid plan.)

You can also check our guide for additional information: Dev Mode for admins

Hope this clarifies!