Group Owners Manual

An owner or moderator can include a display name when they invite or add a member to the group.
Members can add or modify their display names in their Groups.io account profiles or in their profiles for individual groups. (See Customizing your account profile and group profiles.)
Owners and moderators can edit the Display Name field in individual members’ records (however, be aware that members can change their own display names as mentioned above).
If the field is empty when a member initially joins (or is added to) the group, the Groups.io system might automatically populate the field the first time the member emails a message to the group, if the message’s email header contains display name information.