Halliday & Matthiessen (2014: 396-7):
The verbal group is the constituent that functions as Finite plus Predicator (or as Predicator alone if there is no Finite element) in the mood structure (clause as exchange); and as Process in the transitivity structure (clause as representation). …
A verbal group is the expansion of a verb, in the same way that a nominal group is the expansion of a noun; and it consists of a sequence of words of the primary class of verb. If we consider has been eating just as a word sequence, it contains a lexical verb eat, which comes last; a finite verb has, which comes first; and an auxiliary verb been, which comes in between. No other ordering of these three components is possible.
As with the nominal group, we can express this both as an experiential and as a logical structure. Because there is very much less lexical material in the verbal group — only one lexical item, in fact — the experiential structure is extremely simple; and most of the semantic load is carried by the logical structure, including the tense system.