here is the question:

Hi Ulfat ,

With the 'futur antérieur' the context is important ...

Ils auront été ensemble = They will have been happy together 

For instance if you said:

"Dans leurs vieux jours ils pourront se rendre compte qu'ils auront été heureux ensemble" , indicating clearly that the action will be completed in the future...Looking back in their old age, they will realise that they ....

Another example - 

D'ici l'année prochaine ils auront dépensé tout l'argent qu'ils ont gagné au loto = By next year they will have spent all the money they have won in the Lottery .

It is not a tense that is frequently used and can sound weird in English (in my opinion)...

Hope this helps!