Allocations > Members > Add Member: Change “User ID” input field to “Email Address”

**In Allocations > Members > Add Member**, the input field is currently labeled/requesting a User ID. However, members are typically added using their email address.

The input field label, placeholder text, and any related validation should be updated to use Email Address instead of User ID to align with the expected user workflow and avoid confusion.

### Expected Behavior
- Input field label should be Email Address
- Placeholder should indicate an email address (e.g., user@example.com)
- Validation should accept valid email addresses

### Current Behavior
- The form displays User ID as the input field for adding members.