Dear All,
Subforms help you add and track data related to your CRM records. We've always supported most of the commonly used field types in subforms, so as to cater to your business requirements. Now we're happy to extend support to a User fields. Here's a detailed look at this much-awaited enhancement and how it can help your business.
User fields in subforms
You can also add User fields to subforms to associate your CRM users to particular entries in your subforms. Additionally, you can add fields like user email ID or other associated details available on the User details page. However, it doesn't give record access to the associated users. By default, Record accessibility provision is not applicable for user fields in subform.

For example, the subform seen above for Tent Category includes details for camping tents, rooftop tents, and similar tent details, like model number, tent type, and so on. The same subform might also have a user field for the product engineers. Whenever a tent entry is made, the respective product engineer could be associated with that subform entry so that whenever customers ask for technical details, a support person can identify the product engineer they need to contact by checking the User subform field. Additional details about the user/product engineer, like their email IDs, years of experience, and Skype details can be associated with the subform entry as well.
While adding entries to the subform, once the user records or, in this example, the product engineer is associated, related fields like their email or Skype ID fields are updated automatically for the particular subform entry.
The usual limit of using lookup fields in a subform is five. You can use lookup fields or user fields in any combination, up to five fields total.
The user fields aren't supported in subforms for Inventory modules. R
ead more about building a subform here.
Thanks!
P.S. This enhancement is available for all users in all DCs.