This article describes how to synchronize Request participants field on Jira on-premise Service Desk, users are matched by email.
Source Side
Outgoing sync
replica.customFields."Request participants" = issue.customFields."Request participants"
Destination Side
Incoming sync
issue.customFields."Request participants"?.value = replica.customFields."Request participants"?.value?.collect { it ->
nodeHelper.getUserByEmail(it?.email)
}