Adds a new ad hoc group and its members to an existing transaction.

An ad hoc group allows multiple signers to be grouped together, where only one member
needs to sign on behalf of the entire group.

The request body must be an array of Role objects:

1. Individual member roles (regular signers) - these become potential group members
2. The ad hoc group role with a special signer that has:
   • signerType: "AD_HOC_GROUP_SIGNER"
   • group.members: Array referencing the member role IDs

Example structure:
[
{
"signers": [{ "firstName": "John", "lastName": "Doe", "email": "[email protected]" }],
"id": "member-role-id-1"
},
{
"signers": [{ "firstName": "Jane", "lastName": "Smith", "email": "[email protected]" }],
"id": "member-role-id-2"
},
{
"id": "ad-hoc-group-role-id",
"signers": [{
"firstName": "Legal Review Group",
"signerType": "AD_HOC_GROUP_SIGNER",
"group": {
"members": [
{ "userId": "member-role-id-1", "memberType": "AD_HOC_GROUP_MEMBER" },
{ "userId": "member-role-id-2", "memberType": "SIGNER" }
]
}
}]
}
]