It's common for family members to pay for the membership of another family member without actually being a patient themselves. Setting up this situation in Hint is easy, and in fact there are two ways you can accomplish this.
First Way
Find or create the patient you want to pay for the membership.
Click 'New Membership' (Right-hand side of the "Memberships" section)
Add the patients you want to be covered by the membership by clicking '+ Add Adult' or '+ Add Child'
Click 'Remove Patient' on the payer of the membership.
Add whatever else you like to the membership, just like normal. Go here for more on that.
Click 'Create'
Second Way
Find or create the patient who you want to be active on the membership (not the payer)
Click 'New Membership' (Right-hand side of the "Memberships" section)
Click 'Change Payer'
Search for the person whom you would like to be the payer
Add whatever else you'd like to the membership, just like normal. Go here for more on that.
Click 'Create'
That's it! You've now created a membership paid by someone who's not covered by the membership.