Tab Bar is awesome and the recently added features to work with tab
groups made it even better. I know it's still a working in progress, but
I would like to suggest some minor improvements:

1. Add an initial input for the 'completing-read' of
'tab-bar-change-tab-group', so when we type 'C-x t G' to set/unset the
group name, the minibuffer contents is filled with the current group
value. A possible downside is that pressing TAB may not show the
Completions list, in case the current value does not match any item of
the completion list provided.

2. Add new faces for group tabs. The idea is to differentiate these 2
cases:
  2.1. Tabs from the current group X tabs from other groups
  2.2. Regular tabs X collapsed group tabs

This is the default appearance: