Added handle to queue nonbillable users from email

What does this MR do and why?

Fix for handling user invite not getting queued when tried with email_id.

Fixes: [BE] bug: inviting an existing user via email w... (#485076 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Invite Guest Email.mov	Screen Recording 2024-09-19 at 8.10.41 PM.mov

How to set up and validate locally

1. Have an Ultimate License, and simulate Ultimate Self Managed setup (export GITLAB_SIMULATE_SAAS=0 gdk restart)
2. Enable the Setting `Gitlab::CurrentSettings.update!(enable_member_promotion_management: true)
3. Enable the Feature Flag Feature.enable(:member_promotion_management)
4. Create promotion requests:
   1. Visit a group as an owner (e.g. https://gdk.test:3443/groups/gitlab-org/-/group_members)
   2. Click on invite members button
   3. Find a member. This member should have GUEST as the highest role in the hole instance.
   4. Invite them as any role higher then GUEST
   5. They should not be added and should get queued for promotion
5. Try the same thing for the same user using their email_id
   1. Previously:
      1. They would get invited on the higher role
   2. Now:
      1. They should also get queued for promotion