'altogether' and 'all together' : the difference

Altogether means 'with everything considered'.
All together means 'collectively'.

+ Altogether:
The adverb 'altogether' means 'wholly', 'to the full extent' or 'with everything considered'.

Examples:
"I left him altogether convinced that the project will end on time."
"Altogether Mark earns more than his cousin."


+ All Together:
The term 'all together' pertains to when a group acts or is acted upon collectively. (Of note, there can be other words between 'all' and 'together'.)

Examples:
"I want you to sing all together." (I want you all to sing together.)
"The soldiers stood all together waiting for the plane." (The soldiers all stood together waiting for the plane.)
"I would like to see you all together." (I would like to see all of you together.)

http://www.grammar-monster.com/easily_confused/altogether_all_together.htm