1. THE PHARMACEUTICAL SOCIETY OF...
2. PHARMACEUTICAL SOCIETY OF
WHICH ONE OF THESE IS CORRECT TO USE IN INVITATION FOR INVITING OR TO QUOTE IN A INVITATION
The question was the use of the word "The" rather than the use of capitals.
When I'm proofreading a company/group name I go straight to their website and confirm their own usage.
In this case I presume you mean the Royal Pharmaceutical Society of Great Britain (the Royal was added in 1988) or Royal Pharmaceutical Society (as it's generally known).
They do not use the definite article in their name, so don't bother with the 'The'.