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There must be three fully joined members of your organization to be able to create an Approval Group; three is the minimum number for required to create an Approval Group.
Approval Groups can also consist of more than three people.
For a three person group, two out of the three members of that group will need to approve any activity that group gets invoked for.
For groups larger than three, you have more flexibility in setting the number of approvers. For example, in a four person group you can set the number of approvers to two or three; for a five person group you can set the number of approvers at two, three, or four; etc.
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