Hi @Nikhil2 :
If you want to get a SharePoint Site Group's members,I suggest you insert a flow into your app. I've made a test for your reference:
1\First,get the group id

2\Create a flow:

Schema
{
"type": "object",
"properties": {
"d": {
"type": "object",
"properties": {
"results": {
"type": "array",
"items": {
"type": "object",
"properties": {
"__metadata": {
"type": "object",
"properties": {
"id": {
"type": "string"
},
"uri": {
"type": "string"
},
"type": {
"type": "string"
}
}
},
"Alerts": {
"type": "object",
"properties": {
"__deferred": {
"type": "object",
"properties": {
"uri": {
"type": "string"
}
}
}
}
},
"Groups": {
"type": "object",
"properties": {
"__deferred": {
"type": "object",
"properties": {
"uri": {
"type": "string"
}
}
}
}
},
"Id": {
"type": "integer"
},
"IsHiddenInUI": {
"type": "boolean"
},
"LoginName": {
"type": "string"
},
"Title": {
"type": "string"
},
"PrincipalType": {
"type": "integer"
},
"Email": {
"type": "string"
},
"Expiration": {
"type": "string"
},
"IsEmailAuthenticationGuestUser": {
"type": "boolean"
},
"IsShareByEmailGuestUser": {
"type": "boolean"
},
"IsSiteAdmin": {
"type": "boolean"
},
"UserId": {
"type": "object",
"properties": {
"__metadata": {
"type": "object",
"properties": {
"type": {
"type": "string"
}
}
},
"NameId": {
"type": "string"
},
"NameIdIssuer": {
"type": "string"
}
}
},
"UserPrincipalName": {
"type": "string"
}
},
"required": [
"__metadata",
"Alerts",
"Groups",
"Id",
"IsHiddenInUI",
"LoginName",
"Title",
"PrincipalType",
"Email",
"Expiration",
"IsEmailAuthenticationGuestUser",
"IsShareByEmailGuestUser",
"IsSiteAdmin",
"UserId",
"UserPrincipalName"
]
}
}
}
}
}
}
3\Share the flow


4\Add the flow into your app

5\Add a button and set it's OnSelelct property to:
ClearCollect(GroupUsers,Bof1214New.Run().d.results)
6\Click the button added in step 5 and use this formula to determine whether the user is in the specified SP Site Group
User().Email in GroupUsers.Email
Best Regards,
Bof