The word "altogether" means something is whole or complete. Altogether, the warning signs gave me the impression the edge of the cliff was a dangerous location.
The two-word phrase "all together" indicates that people are gathered in a group. Make sure the children are all together before we leave the museum.
Remember it this way. If it's complete, then the words are one complete piece, altogether. If you have to group the two words together, they stand for a grouping of people, all together.