How to list all the members of a group and store them to a local variable to show them in a grid also select multiple members to delete and add the members, using groupmember function?
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You can get a list of all members of a group with the function a!groupMembers(). Then you can use this array to serve as the data basis for a grid. I'm unclear what you mean by "delete and add the members", but you should be able to do various functions all on-form using controls in the grid (like rich text column values with dynamic links perhaps).
So, by delete and add the members means is, after listing all the members in grid, I wants to add/remove multiple members from within the grid.
To add new members, I'd go with a person picker below the grid where you can select an individual user then a special button nearby which, upon clicking, adds them to the group. I would not attempt to do this "in the grid".
To remove individuals, I'd suggest creating a column with a rich text icon and a dynamic link calling the function a!removeGroupMembers() on the username referenced in that row. You could potentially also enable selection of multiple rows and removing all selected members via a button click, however I'd only go with this approach if the row-by-row solution is *completely* unacceptable.