Fix autocomplete of members of group via group share

What does this MR do and why?

Fix autocomplete of members of group via group share

Related issue: #36069 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. As admin, create a public group
2. Create a new private, top level group
3. Add some users as a member of the private group (say bob)
4. Go to Members page of the public group. Invite a group, select the previous private group
5. Login, or impersonate as another member of the public group
6. Create a new milestone in the group. Try to autocomplete @bob in the milestone description.

MR acceptance checklist

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