As the host of a studio session, you have the right to add or remove participants.
As soon as a participant opens the link that you sent him before, he can log into your session after entering his user name, just like you did before.
From your point of view, the new user appears under the studio interface and above the controls (for the camera, microphone, etc.). If you now hover your mouse over his portrait, a button labeled ADD will appear. With this button you "let him in". Then switch the layout to "double view", the 2nd button from the left. Now both you and your 1st guest are visible.
If you want to remove a user from the session, you do it the same way, only now the button doesn't say ADD anymore, but REMOVE.
Note: You as the host have to add yourself first also by clicking on ADD.