Altogether, there are 1 Erika + 3 siblings + 3 friends = <<1+3+3=7>>7 people drawing outside.
The group has a total of 7 people * 3 pieces of chalk each = <<7*3=21>>21 pieces of chalk altogether.
Before Erika’s mom helped, they had 21 total pieces of chalk - 12 pieces of chalk Erika’s mom brought = <<21-12=9>>9 pieces of chalk.
Then adding the chalk that Erika lost shows that the group originally had 9 pieces of chalk + 2 lost pieces of chalk = <<9+2=11>>11 pieces of chalk.
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