Future proche in the past followed by infinitive

Alison, 

the first use of vous is the subject of the sentence ( and of the full verb construct ‘ alliez vous rejoindre ‘ ). 

Look at present tense to more easily follow -  ‘ vous vous rejoignez ‘. 

The first vous is the subject pronoun. 

The 2nd vous is the reflexive pronoun, which acts grammatically also as the direct object of the verb ( as the subject and object of the verb are the same entity ). 

Also, this structure may be easier to follow with another example from the lesson 

        “Oh là là ! Qu'est-ce qu'ils allaient s'ennuyer ! “ 

In this example, the subject pronoun - ils - and reflexive pronoun ‘ se/s’ ‘ refer to the same entity. Being different words however makes the role of each clearer. 

Reflexive verb or not, in any construct with a conjugated verb followed immediately by an infinitive, the objects of the infinitive goes directly in front of the infinitive as per the links below.

 A non-pronominal verb example, this time with both direct and indirect objects.

              The present ?  You were going to send it to him.

              Le cadeau ? Tu allais le lui envoyer.

( Colloquially the ‘ le ‘ may be dropped in the 2nd sentence, but the example is grammatically correct )

Position of French Object Pronouns - with infinitives 

Conjugate reflexive verbs in the near future in French using aller + infinitive (Le Futur Proche) 

https://www.lawlessfrench.com/grammar/reflexive-pronouns/

Thanks Maarten

I’m aware of the various grammatical constructs of the probouns(subject, object, reflexive…). The sentence I queried seemed to me to be inverted as in a question. Thank you for the example with s’ennuyer, which I now see has the exact same structure. This one didn’t bother me as, to my brain, it was in the infinitive.  So, basically you match the reflexive pronoun with the subject pronoun, even in the infinitive?

Yes - the use of the pronouns and the order of any object pronouns remain the same.