One of the possible translations given for the last sentence of the exercise is 'Peut-etre qu'on peut s'entraîner ensemble ?' Does not s'entrainer imply a joint activity and so 'ensemble' is not required?
Freeform Writing Exercise B1
"s'entraîner" is a reflexive verb but it doesn't imply a joint activity unless you add a marker (like "ensemble").
I hope this is helpful.
Bonne journée !
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Test your French to the CEFR standard