'DAO method - requires DAO reference
Public Function IsUserInGroupD( _
       ByVal UserName As String, _
       ByVal GroupName As String _
       ) As Boolean

   Dim usr As DAO.User
   Dim grp As DAO.Group

   Set usr = DBEngine.Workspaces(0).Users(UserName)
   For Each grp In usr.Groups
       If grp.Name = GroupName Then
           IsUserInGroupD = True
           Exit For
       End If
   Next grp

End Function

'ADOX method - requires ADOX reference
Public Function IsUserInGroupA( _
       ByVal UserName As String, _
       ByVal GroupName As String _
       ) As Boolean

   Dim cat As ADOX.Catalog
   Dim usr As ADOX.User
   Dim grp As ADOX.Group

   Set cat = New ADOX.Catalog
   Set cat.ActiveConnection = CurrentProject.Connection
   Set usr = cat.Users(UserName)
   For Each grp In usr.Groups
       If grp.Name = GroupName Then
           IsUserInGroupA = True
           Exit For
       End If
   Next grp

End Function

Examples of use, in the Immediate window ...

? isuseringroupd(currentuser(),"Admins")
True
? isuseringroupd(currentuser(),"Test")
False
? isuseringroupa(currentuser(),"Admins")
True
? isuseringroupa(currentuser(),"Test")
False