@MasaYR - this is my method, see if you can leverage this for your needs.
1. In the Items property of the ComboBox control inside the DataCard, I would use:
Office365Groups.ListGroupMembers(
"groupID888888888888888",
{'$top': 999}
).value
2. In the Update property, use:
ForAll(
ComboBox1.SelectedItems As _selections,
{
DisplayName: _selections.displayName,
Claims: "i:0#.f|membership|" & Lower(_selections.mail),
Department: "",
Email: _selections.mail,
JobTitle: "",
Picture: ""
}
)
3. In the DefaultSelectedItems property, enter:
If(
!IsBlank(ThisItem.'Your Multi Choice People Field'),
ForAll(
ThisItem.'Your Multi Choice People Field' As _selections,
First(
Filter(
Office365Groups.ListGroupMembers(
"groupID888888888888888",
{'$top': 999}
).value,
mail = _selections.Email
)
)
)
)
Do not make any other modifications to the DataCard.
Note
I find the performance of using SharePoint user groups as poor when applying them into the Items property of ComboBoxes. You can literally see the loading dots appearing at the top of the screen while the Combobox is loading in the selected items.
See a blog I wrote below for a faster alternative. Note this solution is specific to single choice people fields, but the same concept can be applied to multi-choice people fields:
https://powerusers.microsoft.com/t5/Power-Apps-Community-Blog/Filter-SharePoint-single-select-People-Picker-based-on-another/ba-p/2235959
