refactor: add members

[brech1]

Description

This PR optimizes the add_members function in src/group.rs by changing the input from Vec<Element> to &[Element], reducing memory overhead by avoiding vector cloning. Tests in src/group.rs and tests/group.rs are updated to use slice syntax.

It also fixes clippy and docs warnings.

• What kind of change? Performance optimization (refactoring)
• Current behavior: add_members requires an owned Vec<Element>, causing unnecessary cloning.
• New behavior: Accepts &[Element] slice for efficient reference passing.
• Breaking change? Yes, users must update calls to pass slices instead of vectors.

Related Issue(s)

• Resolves Optimize add_members in Group #9

Other information

All unit tests pass locally with updated slice syntax. No additional documentation changes needed.

Checklist

• I have read and understand the contributor guidelines and code of conduct.
• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have run cargo fmt without getting any errors.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.