Fit in:
 1. to be accepted by a group of people because you aresimilar to them
 exempl:  She fitted in with her new colleagues straight away. 

2. -  to have enough time to deal with someone or something

 exempl:  He fits in his work around his family commitents. 

 3. - to have space for someone or somethin

 exempl:  How many lines can you fit in per page? 

Fit in with something:  

-to be correct, suitable, or sensible in relation tosomething else

 exempl:  His explanation didn't fit in with what I saw.

Fit in : 

- go together; "The colors don't harmonize"; "Their ideas concorded"