I need to install @openzeppelin/community-contracts to try MultiSignerERC7913 contract:
How can I install the package? I couldn't find it in npmjs.
Hi, welcome to the community! 
I think you can use forge install OpenZeppelin/openzeppelin-community-contracts, for more details, you can have a look at here: https://docs.openzeppelin.com/community-contracts
Notice: This repository has not been audited and may introduce breaking changes at any time.
Yes, but I found an alternative. I needed to import ERC7821.sol contract and that is included in @openzeppelin/community-contracts. Instead, I used draft-ERC7821.sol that is included in @openzeppelin/contract. So, I can install @openzeppelin/contracts from npmjs and import:
@openzeppelin/contracts/account/extensions/draft-ERC7821.sol
into my contract code.