You need to ask the future wallet co-signers to get their signer key for you first. Here is how:
Make sure you are on the latest version of Argent X wallet. Argent X is available as an extension in Chrome Web Store, Microsoft Edge and Firefox Addons.
Open your Argent X and go to the accounts list.
Click + and select Multisig account > Join existing multisig.
Copy the pubkey (it will not grant access to your Argent X wallet, so it is safe to share with your co-signers) and sent it to the account creator.
Once the account creator has the keys, they can create a new multisig and add these keys as co-signers.
After the multisig account is created you will need to activate/deploy the account on-chain to transact with it.