I'm totally guessing that since "los" is a shorter(less syllables, less letters) way of expressing "a ustedes" Spanish speakers naturally more often use "los" instead of the sentence with the additional "a ustedes". The "a ustedes" here is optional for clarification that it not "a ellos". As a result of this it maybe essentially a rule or so common that it is a de facto rule that the "los" has to be here.
In English "will" or future tense can be used for any future, but "...going to..." can only be used for the near future or for planned future. So, "will" should be accepted anywhere "going to" is, but "going to" cannot be used everywhere that "will" is. However, for Duolingo purposes they originally did not accept "will" for "going to", because they were trying to teach that particular version of the future. Some people reported "will" as an alternate, but reporting is done sentence by sentence and I am not sure if it is allowed everywhere at this time.
On several translators "vamos a seguirlos a ustedes" translated as ''We are going to follow them to you''
But, (by changing the position of ''los'') ''Los vamos a seguier a ustedes'' translated as ''We are going to follow you''
In both cases ''los'' was the direct object, but in the first case ''ustedes was considered an indirect object and in the second case ''ustedes'' was considered as clarifying/renaming the direct object.
This is an error on the part of the translator, because there must be an indirect object pronoun any time there is an indirect object involved. So, "a ustedes" can only be clarifying the direct object pronoun here. For it to be "We are going to follow them to you." the Spanish would have to be "Se los vamos a seguier a ustedes." or "Vamos a seguírselos a ustedes." at the very least and perhaps "Se los vamos a seguier a ustedes a ellos." or "Vamos a seguírselos a ustedes a ellos." (indirect object is placed before direct object when both are in a sentence.) Yet that would still be confusing to some, I think there would be context and you would likely only say "Vamos a seguírselos."
They is ellos or ellas if only women are included. The unstressed obligatory direct objectform is LOS, the stressed form is A ELLOS/ELLAS. The los in the sentence is the unstressed direct objectform of the direct object ustedes and has nothing to do with the subject nosotras. From the fact that Duo uses NOSOTRAS we can conclude that there are no men in Duo's we