Fix incorrect member count during group creation and modification

[Cale146]

Resolves issue #13206 where the member count displayed was inaccurate when creating or modifying groups. The member count now reflects the correct number of participants when selecting/deselecting.

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• I have read how to contribute to this project
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Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Motorola Moto G Power 5G, API 34
• Virtual Device Pixel 9 Pro, API 35

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #13206 syntax

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Description

Previously, the displayed member count in the 'X members' message was incorrect when adding/removing members from both new and existing groups. This PR ensures the correct count is shown at all times.

[greyson-signal]

Thank you! This will go into 7.36

[Cale146]

Thanks @greyson-signal.