What type of pronouns is joining words?

Pronouns are not joining Words. A pronoun is a Word that takes the place of a noun in a sentence.

Example sentence: When George got to 19th Street, he got off the train (the pronoun 'he' takes the place of the noun 'George' in the second part of the sentence)

Joining Words are conjunctions and prepositions.

A conjunction is a Word that joins two Words, two groups of Words or two parts of a sentence.

Examples of conjunctions are: and, but, or, nor, for, yet, so, although, because, since, unless, etc.

Example sentence: Jack and Jill went up the hill so they could catch the school bus.

A preposition, usually coming in front of a noun or pronoun, expresses a relation to another Word or element. A preposition introduces a prepositional phrase.

Examples of prepositions are: across, at, beside, by, for, from, in, of, on, over, per, through, to, etc.

Example: The cake on the table is forthe children.