Sunject verb agreement basics

The “and” is part of the modifier which is not the main subject. The modifier modifies the main subject “Jan Holder”, who is singular. The verb associated to the main subject must also be singular then.

I haven’t contributed a lot to the SC forum so not sure if “middleman” is commonly used, but if you are referring to middleman in the way that MGMAT does than “of” is not a middleman here. The main subject is “group of managers”, not just “group”. I agree with Economists explanation above on why verb is plural but didn’t pick that up straightaway.

AND is not used as a "additive phrase" in this sentence. Lets see why not:

Her partner and business associate, Jan Holder, [strike]distribute[/strike]/distributes the daily mail. - In this sentence, the part underlined plays a role of a modifier, which modifies the (singular) subject Jan Holder. Here word AND is not connecting two different subject, but is connecting two different qualities or roles of the singular subject Jan Holder. This is the reason we need to use singular form of the verb.

Now consider this example:
Her partner, Jan Holder and her business associate distribute/[strike]distributes[/strike] the daily mail. - Again, in this sentence, the part underlined plays a role of a modifier, which modifies the (singular) subject Jan Holder. But now word AND is connecting two different subjects, Jan Holder and one other business associate. This is the reason we need to use plural form of the verb.

As Economist has already said correctly in his previous post, the sentence is referring to people within the group. The subject of the sentence here is not the group itself, but the people of group who arrived one by one.
Lets rephrase the sentence to make is more clear.

Original sentence: The group of store managers have/has begun to arrive, one by one.
Rephrased sentence: One by one (the members of) the group of store managers have/[strike]has[/strike] begun to arrive. - The subject of the sentence here is clear now . So we need to use plural form of the verb, have.

Hope this clarifies your doubts.
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In the Second Question

Quote
" The staff is/are eating at their desks to make Tuesday's news deadline. "
Unquote

The Staff is singular. However the pronoun "their" is plural. So I believe, it will not be correct to conclude that "is" will be used.

Can someone please explain why 3. is plural and not singular? I thought "Group" is a Collective Noun, for instance "Our army of a hundred thousand soldiers is attacking the enemy. BTW I have this example from the MGMAT Guide.

Thanks a lot.

Correct for question 3 answer should be singular " has ".