How to initialize PeopleEditor control?

You can use CommaSeparatedAccounts property for users. But this property doesn’t work for groups.

SPGroup

PeopleEditor peopleEditor = ...;
SPGroup group = ...;
PickerEntity entity = new PickerEntity
{
    Key = group.LoginName,
    DisplayText = group.Name,
    Description = group.Name,
    IsResolved = true
};
entity.EntityData.Add("AccountName", group.LoginName);
entity.EntityData.Add("SPGroupID", group.ID.ToString());
entity.EntityData.Add("PrincipalType", "SharePointGroup");
peopleEditor.Add(entity);

SPUser

PeopleEditor peopleEditor = ...;
SPUser user = ...;
PickerEntity entity = new PickerEntity
{
    Key = user.LoginName,
    DisplayText = user.Name,
    Description = user.Name,
    IsResolved = true
};
entity.EntityData.Add("AccountName", user.LoginName);
entity.EntityData.Add("SPUserID", user.ID.ToString());
if (user.IsDomainGroup)
    entity.EntityData.Add("PrincipalType", "SecurityGroup");
else
    entity.EntityData.Add("PrincipalType", "User");
peopleEditor.Add(entity);