The Buddy System: Seat the newcomer next to a responsible student who speaks the same home language.  This will allow the newcomer to keep up with the class (as long as the students are only discussing the schoolwork).  However, please try to make heterogeneous groups for certain activities as well; make sure the ELLs are also regularly mixed with non-ELLs when you group, in order to expose them to native English speakers.  
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http://www.colorincolorado.org/article/how-create-welcoming-classroom-environment