all together / altogether
Everyone in one place versus entirely.
All together means all in a group, at the same time or in the same place. Altogether means entirely, on the whole, or in total. If you can separate the all (they arrived all together → they all arrived together), you want two words.
Examples
Use The cousins sat all together on the porch.
Use That is altogether different.
Use The repairs cost $400 altogether.
Avoid That is all together different.