In the recipient areas supervisor and participant, you can choose between the options All and Select individually to select the recipients of the email. However, the display of these two options is linked to the group settings for displaying the member names.
For example, if the option Members can see members in the group configuration is disabled, group participants will only be able to see and select the entry Email to participants in the group mailing section of the course view. In other words, they will only be able to send emails to all members of the group but not to individual recipients. The latter will only be possible if the option Members can see members in the group configuration is enabled.
These settings do not apply to course supervisors and group tutors, as these user roles can always select individual participants as recipients.