Usually, we disregard prepositional phrases when deciding number agreement between a subject and verb. The most obvious answer to your original question is (a): "There was a series ...". The subject is "series". The prepositional phrase "of meetings" is adjectival, describing the series.
Unfortunately, despite Bazza's harsh and ignorant reply, the question is not always that simple. There are a lot of exceptions.
Case in point: Going by what I said earlier, that last sentence should have been:
- There is a lot of exceptions.
Here, "a lot of" is a set phrase that is interchangeable with the single word "many":
- 1: Many people think that prepositional phrases should be disregarded when determining subject and verb agreement. (correct)
- 2: A lot of people think that prepositional phrases should be disregarded when determining subject and verb agreement. (correct)
- 3: *A lot of people thinks that prepositional phrases should be disregarded when determining subject and verb agreement. (incorrect)
The subject is "people", not "lot".
Sometimes, collective nouns in the singular form can take either a singular or plural verb, depending on context:
- 4: The team is the best that Boston has had in forty years.
- 5: The team are all wearing plaid socks.
The difference here is that in (4), the team is being described as a unit, whereas in (5), it refers to the individual members.
- Qa: A group of us is going to London.
- Qb: A group of us are going to London.
I would say that both of these are acceptable, technically, but neither sounds natural.
David, Gustavo, would you care to contribute?