First off, you should really start thinking in terms of addresses rather than accounts, especially when doing block chain analysis. See here: What is the difference between accounts and adresses?

You should also start thinking about transaction inputs not in terms of addresses, but rather unspent transaction outputs (UTXOs). Each UTXO will generally have an address associated with it, but in the case of multisig, may have several addresses in a single UTXO. So in this case, even a single input to a transaction has more than one person associated.

There are also the cases of coin mixers, web wallets, and exchanges, which often create single transactions that include many inputs and many outputs, all coming from and going to different people.

So to answer the question, no, you cannot assume that all addresses used as inputs to a transaction come from the same person.