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/