Sending a general invoice:
Go to Finances > + New Invoice.
Fill in the required fields and information to send out an invoice whether it is to a group or to an individual.
You can then send out the Invoice via email and will allow you to follow it up, sync with Budget etc.
Sending a membership related invoice (e.g. renewal reminder)
Go to Membership
Select New Membership from the green drop down
Pick the Membership Level
Select the Contact/Member
Choose "Needs an Invoice"
Please note: If you're wanting to send an Invoice relating to Memberships please do so through the Memberships section. If you do it though the Finances section it will not link the Contact to the Membership.