For example, if I want to say an account of Google+, is it the same as say a Google+'s account?
In other words, are possessives formed in the same way as always, no matter whether the word ends in a letter or a symbol?
Words that end in symbols are not properly formed according to traditional rules of English spelling. However, that being said, I'd say you can treat the symbol as just part of the word, and pretend the + is just a letter.
I'd caution you though that your example
doesn't make sense to me. First of all, there is only one Google+ so you wouldn't say "A Google+". There is only "The Google+". If you are using "Google+" as a modifier for "account" then you'd say "A Google+ account". This is perfectly fine and normal. It's easy to write, read, and even say out loud. It is grammatical and unobjectionable, beyond whatever hatred you may or may not have for commercial names.
Some examples of people using "Google+" as if it didn't have any unusual symbols or "letters":