Implement referral system for group growth

[big14way]

Track referrals and reward members who invite others to join groups.

Acceptance Criteria:

• Add optional referred_by parameter to join_group
• Track referral count per member
• Add get_referral_count(member) view function
• Emit referral event for off-chain reward distribution

Files: contracts/sorosave/src/group.rs, contracts/sorosave/src/types.rs, contracts/sorosave/src/storage.rs

[Muhammadhassan1226]

@big14way This maps closely to work I've already done in production — I built an end-to-end affiliate/referral system on Vonza where we track unique clicks via vonza_visitor_id + ref_id per product type, calculate commissions, and trigger automated emails on conversion events. So the core logic here — tracking who referred whom, counting referrals, and emitting events for off-chain reward distribution — is very familiar territory for me.

My approach for this:

Add referred_by: Option

to the join_group parameters in group.rs
Store referral relationships and counts in storage.rs with a clean key structure
Implement get_referral_count(member) as a read-only view function in group.rs
Emit a ReferralEvent on successful join with referrer + referee addresses for off-chain reward distribution
Update types.rs with any new structs/enums needed
The Vonza system also taught me edge cases to handle — like preventing self-referrals, duplicate referral tracking if someone rejoins, and making sure the referral only counts on a successful join (not just an attempt).

Can get started immediately. Please assign me!