I would suggest you open the AddMember form as a popup, modal form. This way, the AddMember form appears as a dialog box and doesn't impact the record you're editing. Code would look something like this (assumes your Add Member button is named "cmdAddMember" and that your Add Member form is named "frmAddStaff"). You would place this code behind the click event of the cmdAddMember button:
Code:
Private Sub cmdAddMember_Click()
DoCmd.OpenForm "frmAddStaff", , , , acFormAdd, acDialog
'Add code here to requery the drop down list(s), such as:
'Me.cboStaffMembers.Requery
End Sub