Saving User array into a child field CDT, and writing it to the DB along with the parent CDT using Write to multiple data store entity smart service

Hi,

I have a CDT meeting_details, in that I have a field attendees with type CDT

 

Child CDT named Attendees is the foreign key in the parent table

In Interface I have used User picker, and collecting multiple users into text format, further I will be using those users to create a group and users into that.
When i am trying to use write to multiple data store entities and giving a single user in the interface, the parent as well as child table is getting updated, but when there is a array of users from interface, then it is populating this error.

I am providing here the code for configuring of write to multiple data store entity smart service 
= {
{
entity: pv!meetingDSE,
data: { pv!meetingDetails }
},
{
entity: pv!attendeeDSE,
data: { pv!attendeeData }
}
}



I am providing here the process variable snippet

Can anyone help me out from this issue

Thanks in advance

Prithvi