We are trying to cut down out forms.Currently we have
First Name
Last Name
Company Name
Phone number and email
Are all these fields necessary? Do we need to captaure email if we already have it (we're emailing them)
I think a good practice here is to prefill the fields so that users do not have to fill them out a second time.
At minimum you will need email address.
"Necessary" fields are a business decision.
Do you need Company Name? In B2B, you absolutely do. If you're not doing business with businesses, you still may want it as a demographic pointer; or maybe you don't because it's interfering with conversions. We can't say for sure.
Do you need Last Name? Similar set of questions. Are you doing direct mail? Then you need it now. Is Sales comfortable calling people and saying, "Hi, may I speak to Elizabeth?" and getting details later in the call? Then maybe you don't.
Phone Number, same deal.
The only one that an outsider can (somewhat) objectively evaluate is Email. If everyone hitting your LP is coming from an email, you already have an email, but if they have a better one you want to know it. I recommend having a secondary field (like SelfUpdatedEmail) and have that field be the one on the form, as opposed to the primary Email field. This way they are updating a field on the same lead record. If you expose Email to them, they will create new records. That's bad.
Sanford is correct that it's a business decision, but you should also consider what is required for your CRM if you are doing a CRM sync. Our Salesforce requires first name, last name, company and email address or the lead will not be created.
Good point. You can still give the field a default value without showing the field to the user, though.