Just an FYI because this was a tricky one to troubleshoot. We had many users not showing up in the participants tab even though they were in the same OU.
If a user in AD does not have an email address, they will not show at all in the LDAP lookup for participant users in forms. They do show in ldap explorer, but just not the forms product.
More and more companies are switching to outside email providers like Google and not re-linking the sync to AD.