What are the differences between gerund and present participle?
Is the following a case of present participle? People joining the association will have to pay.
A gerund functions as a noun, and so as the subject or object in a clause or phrase. A participle functions as an adjective or adverb, and modify a noun or phrase.
Many grammarians find the distinction between participles and gerunds to be unnecessary and say that they are not really different in modern English. You can follow these experts and say "it is an -ing word" and leave it at that.
Your sentence is an example. You could say that there is a gerund acting as a subject "People joining the association", or you could say that the participle phrase "joining the association" modifies the noun "People".
If you look at the meaning of the verb "pay" it has to apply to "people" and not "joining". So if you have to choose, choose participle.