Learn English – What’s the difference between grammar and syntax

Subscription implies a service that is paid for on a regular basis. Membership may be paid for, but that is not implied.

I could be a member of the Zooey Deschanel fan club for free, but subscribe to the monthly Zooey Deschanel magazine for $20 a year, for example. A man can dream...

Also, one can be described as a member of organisations that actually pay you, like the armed forces or emergency services.

Members of a subscription system are generally referred to as subscribers, but this is context sensitive. Your friend could simply refer to them as customers, as that's the usual word for someone paying for goods or services.

These words are pretty similar and have only subtle differences and in spoken language many people might not be careful enough to use each of the words correctly. However I think the explanation from Longman Activator Thesaurus is quite helpful:

purpose: what you want to achieve when you do something; the reason you do or plan something, and the thing you want to achieve when you do it: The games have an educational purpose.

aim: something you hope to achieve by doing something: The main aim of the plan was to provide employment for local people.

goal: something important that you hope to achieve in the future, even though it may take a long time: The country can still achieve its goal of reducing poverty by a third.

target: the exact result that a person or organization intends to achieve by doing something, often the amount of money they want to get; a particular amount or total that you want to achieve: The company is on track to meet its target of increasing profits by 10%.

objective: the specific thing that you are trying to achieve - used especially about things that have been officially discussed and agreed upon in business, politics, etc. and agreed upon in business, politics, etc.: Their main objective is to halt the flow of drugs. | We met to set the business objectives for the coming year.

ambition: something that you very much want to achieve in your future career: Her ambition was to go to law school and become an attorney. | Earlier this year, he achieved his ambition of competing in the Olympic Games.