Which is grammatically correct and why?
- Members are requested to pay their subscriptions in time.
- All the Members are requested to pay their subscriptions in time.
Both are grammatically correct, but they are used in different contexts.
is a very formal, polite way to give an order. You often hear it on aircraft, e.g.:
This is probably the form that you should be using.
is not usually used to give instructions. You are just describing the situation. E.g.:
This isn't a grammar problem but a semantics question:
I think the default assumption is that they are equivalent, unless it's common and accepted practice in the club for some members to pay their dues late.
I also think the phrase in time should be changed to on time. The former phrase means to do something before something else happens, e.g.,
The latter means to do something before the deadline, i.e., in a timely manner, e.g.,
Using members, the sentence addresses the group of people comprising members as a unit.
Using all members, on the other hand, addresses individual members.
The difference, as Bill Franke has noted, is not one of grammar but semantics. It may be important, say, from a protocol point of view.