Convenir
Concerning the auxiliary to be used, "convenir" (to convene) is conjugated in a literary way on the verb used to construct it: "venir". We then use the auxiliary "être". However, the common use of this verb is to use the verb "avoir".
Nous avons convenu de nous revoir. (forme courante)
Nous sommes convenus de nous revoir. (forme soutenue)
If you wish to use a sustained level of language, the avoir auxiliary is the most appropriate, otherwise the avoir auxiliary should be used.