I was under the impression that even an individual could, if he completes the proof of work first, cause the addition of the block to the blockchain? What am I missing here?
That it could happen that 2 blocks are found at the same time. Which usually causes the network to split into two chains, ( One part of the network is going to build/find a new hash based on block y, and the other part of the network is trying to find a new valid hash based on block X.).
Whoever finds Block Z first creates the longest chain and thus invalidating the shorter one. ( Miners who build on block Y find block Z so now block X ( which is the shortest chain) gets orphaned )= now invalid.
The same principle applies to a 51% attack, since you have majority hashpower you will always (hypothetically) find a valid hash faster then the other chain which has 49% hashpower.
In that case, no individual ( unless the individual owns 51% of the hashpower) is going to be able to "mine" a block/(or rather, - get a block added to the longest/valid chain).
Your title is something else then your question,
Do multiple miners contribute to mining the same block even if they are not in the same pool
Well, yes, if there's only 1 chain, every miner is going to be building upon that chain, which means they'll all be looking for a block ( or rather a hash) that is going to be valid on that chain ( basically the next block?)
I wouldn't use contribute though, it's not like they share their hashpower, but they do indeed share the same goal. -> Finding a block with a valid hash that is valid on the current chain.