What type of pronouns is joining words?

"Joining words" is not a term used for pronouns. Pronouns are words that replace nouns, and they don't "join" anything.

Perhaps you're thinking of conjunctions, which are words that connect words, phrases, or clauses. Some examples of conjunctions include:

* Coordinating conjunctions: and, but, or, nor, for, so, yet

* Subordinating conjunctions: because, although, since, while, if, unless, until, when, where, before, after

Let me know if you have another question!