How can users sign a multi-signed transaction?

Question

Since Asch v1.4 we also got groups. A group is an address owned by more than one user.

According Asch's test code adding a new member requires the signatures of (all) group accounts:

I can create a MultiSigTransaction as follows:

trs = lib.AschJS.transaction.createMultiSigTransaction({
      type: 502,
      fee: 100000000,
      senderId: groupAddress,
      args: [group.newMembers[0].address, 1, 4],
    })

After that i can sign it with the following code:

trs.signatures = []
for (let i = 0; i < 3; i++) {
  trs.signatures.push(lib.AschJS.transaction
    .signMultiSigTransaction(trs, group.members[i].secret))
}

But the above requires the secrets of the group members.

In a production environment i don't know the secrets of the member and should each each account adds it's own secret.

Where and howto store the unsigned Transaction and enable each user to sign it?