Before we can discuss the specifics, it is important to realize that this confusion is part of a bigger issue. In English, there are several words that combine all and a root word to create alroot. Follow so far?
All Together is an adverb that means at the same time or as a group. For example: One, two, three, all together, sing happy Birthday to you and let’s go to the movie at the drive-in all together, it’s much more fun that way! So if you can substitute let us or drop the all and the sentence still makes sense use all together.
Altogether is also an adverb that can mean completely, total or considering everything. For example: today’s deposit was $439 in cash and $588 in checks: $1027 altogether or altogether, it wasn’t such a bad trip, despite the sort of disastrous incident with the beehive.
The Education Bug website offers a mnemonic you can use to differentiate all together and altogether. “Remember that all together – because it’s two separate words – is the one that needs to get into a group and get in sync. This associates the meaning with the spelling to help you remember which of these easily confused words is which.”