I have many users inside the organization and I want to use same ethereum account for all the users , but with different usernames, so it there anyway to do that ?
If they all use the same private key they can use the same account. But Ethereum does not have a concept "username" - that's something that is between your contract and your backend system.
So let them all use the same private key and they will use Ether from the same account. Then it's up to your contract to decide how to distinguish the users - probably a username of some sort. This username needs to be configured in your off-blockchain code.
Multisig (multiple signatures) wallet is a common way to collectively own Ethereum address. Basically, multisig wallet is a smart contract with several owners, which performs actions that collected enough owner approvals.