Insert into usergroup (user_id, group_id, default_group)
 Select user_group.user_id from [user_group]
      join [group] g
      on user_group.group_id = g.id
      where g.name = 'someName';

Insert into usergroup (user_id, group_id, default_group)
Select ug.user_id, 1234 AS group_id, ug.default_group
from [user_group] ug
  join [group] g on ug.group_id = g.id
where g.name = 'someName';