"Sometimes he plays with us."
Translation:Soms speelt hij met ons.
"Bij" means 'at' (but can sometimes be translated to 'with')
"Soms speelt hij bij ons" should be translated to "sometimes he plays at our place" or something like that so there is a difference in meaning between the two sentences.
That's not a correct order. You can use Hij speelt soms met ons and if you want to put some more emphasis on soms you can start with that word, but then inversion takes place and the verb comes before the subject: Soms speelt hij met ons
Also see this explanation: https://www.duolingo.com/comment/3733010