Allow assigning Marketplace signups to a specific group (instead of Default Group)

Problem:
Currently, affiliates who join a program via the Affonso Marketplace are always assigned to the Default Group. This limits flexibility for programs that want Marketplace affiliates to receive a different commission structure than direct signups.

Proposed Solution:
Allow program owners to select a specific group that Marketplace affiliates should be assigned to upon signup, instead of forcing all Marketplace joins into the Default Group.

This could be implemented as:

• A setting in the Affiliate Program configuration (e.g. “Marketplace signup group”), or

• A dedicated “Marketplace Group” selector, with Default Group remaining the fallback.

Use Case:
Some programs treat Marketplace affiliates differently (e.g. lower starting commission, different approval logic, or special terms). Being able to route Marketplace signups into a predefined group would remove the need for manual reassignment and better reflect real-world affiliate workflows.

Current Behavior:
Marketplace signups → always assigned to Default Group.

Desired Behavior:
Marketplace signups → assigned to a user-selected group.