Option 1) copy + pasting their email addresses, separated by new lines or commas. Then select to '+ Add new learners'.
Option 2) uploading a CSV file, by selecting 'Assign learners to cohorts by uploading a CSV file'.
Note: Learners may only belong to 1 cohort. If a learner is already assigned to Cohort A, and you manually assign him to Cohort B, he will automatically be removed from Cohort A.