You can enable the option to make admins and/or team leaders able to approve the enrolment of a user to a session. To enable this option, first click on the Studio tab, then on the Sessions tab:

Selecting the session you want to edit will take you to the Edit session page. Here you can find the option to approve the enrolments, that will be disabled by default:

With that option enabled, you can choose who you want to be able to do the approval:

With the option of team leaders selected, when users enroll in a session, they will see a dialog telling them permission is needed to do that, and they will be able to select one or more team leaders to process the approval:

With the option of admins selected, you will have the choice of making all admins able to approve an enrolment or just specific ones:

It's important to note that when both admins and team leaders are enabled, the decision of the admins will always overwrite the one from the team leaders, so admins will always have the final word over approving an enrolment if they get involved.

Once an enrolment is sent for approval, it can be reviewed from the Session Requests page. You can go there by clicking on the Learn tab, and then the Manage tab:

On the Session Requests tab, you will see all pending approvals. You can approve or decline any of them individually with the buttons at the end of each row, or you can do several at once selecting them with the checkboxes on the left side and clicking the Approve or Decline buttons below:

When an enrolment is declined, a dialog will open asking for a reason:

Once an enrolment is declined, users can try and enroll again for the same session, and the same approval process will apply.