Twice a month I send a piece of our content to contacts from SFDC through Marketo.
I ran a report in SFDC, exported the list, then imported the list into Marketo to ensure that I was emailing the correct contacts.
About 50 of the 300 on that list did not sync with SFDC and will only sync if I create a new lead in SFDC.
Is there an easy way to map these "leads" in Marketo to our contacts that we already have in SFDC?
There isn't any way to force a lead record from Marketo to map to a Contact record that already exists in Salesforce. The question instead is "Why haven't those Contacts synced from Salesforce to Marketo?"
The solution would be to make sure that your sync user can see those Contacts, and then make some update to them, which will sync them to Marketo. Syncing them from Salesforce is going to be the only way to connect them to Marketo, as there isn't any way to connect two records that already exist in each instance.
Thank you so much for your quick response John.
I have troubleshot our SFDC sync and everything seems to be working, other than these 50 or so contacts.
When you say "sync user" are you referring to our SFDC admin?
The sync user would be the Salesforce user whose credentials you've place in Marketo Admin under the Salesforce section. Those credentials allow Marketo to sign into your Salesforce instance, and they determine what permissions Marketo has while signed in.
Just to add to John's comment - Marketo exists as a user in your SF instance, and like with any other user your sfdc admin can limit records it can access through sharing rules, here is a good video explaining sharing rules in more details - ttps://www.youtube.com/watch?v=JEKZ2OjwzS8.