In Timeline View, Grouping by a contact column sets up groups for only the contacts shown in the contact column properties, and not for every contact on the sheet. It should read the contacts on the sheet and group them.
What happens now is painful, as team members are newly assigned to sheets, the timeline does not update to show their assignments by their name, instead it shows their assignments in the "Undefined" group. This is not intuitive, at all!
Also the grouping is in order that the contacts appear in the Column properties, rather than something more meaningful like alphabetical.