1)could + infinitive (es they could go)
2)could have + past participle of verb (ex. they could have worked)
3)could have been (they could have been working)
4)could be (they could be working)
I would use the following terms.
be going modal progressive
have gone modal perfect
have been going modal perfect progressive
Other names are:
go modal with infinitive
be going modal with progressive infinitive
have gone modal with perfect infinitive
have been going modal with perfect progressive infinitive
As you see, I prefer to use the less wordy descriptors.
The term 'continuous' substitutes for 'progressive' in some textbooks.
People are waiting to help.