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# Groups

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## Key points

### Merkle tree/groups relationship

Groups are actually incremental Merkle trees and group members are tree leaves. Since the Merkle tree implementation we are using now is a binary Merkle tree, the max number of members of a group is equal to `2 ^ treeDepth`. We will allow devs to use different tree depths (probably a range from 16 to 32). It could make sense to show the different group sizes a dev could use based on tree depths:

| Tree depth | Group max size |
| -------- | -------- |
| 16 | 2^16 = 65536 |
| 17 | 2^17 = 131072 |
| ... | ... |

Merkle trees are very useful when you want to prove that a leaf is part of the tree without sharing the whole data structure, but only the path from the leaf to the root. Merkle proofs are part the Semaphore zero-knowledge proofs.

### `SemaphoreGroup.sol` contract

Groups can be used by extending the `SemaphoreGroups.sol` contract. It contains 3 internal functions that must be called in the parent contract for group management (`_createGroup`, `_addMember`, `_removeMember`), and 3 public functions to get group information (`getRoot`, `getDepth`, `getNumberOfLeaves`).

The extension contracts are a good example to see how Semaphore groups can be used.

## Suggestions

* The guide could introduce the Semaphore groups by explaining what they are and why they are used.
* The guide could explain how developers can use the `SemaphoreGroup.sol` contract and its functions.