I'm embarassed that I had two Option Explicit statements. I'm learning but have been told this before .

I've added a ComboBox to list the three teams, although now I have a 'Variable not defined' here:-

Private Sub ComboBox1_Change()
I'm sure I'm doing something silly to get this error.

Option Explicit

Private Sub EnterBut_Click()
    Dim oTeamMembers As Range
    
    'Check if a team has been selected
    If ComboBox1.ListIndex = 0 Then
        MsgBox "Select Team", vbCritical, "Triage Hub"
        ComboBox1.SetFocus
        Exit Sub
    End If
    
    'Check if team members have been selected
    If TextBox1.Text = "" Then
        MsgBox "Select Team Members", vbCritical, "Triage Hub"
        TextBox1.SetFocus
        Exit Sub
    End If
    
    'use FillBM function to write bookmarks
    FillBM "TeamMembers", TextBox1.Text
    
    Set oTeamMembers = Nothing
    Unload Me
    lbl_Exit
    Exit Sub
    
End Sub

Private Sub ComboBox1_Change()
    Select Case ComboBox1.ListIndex
        Case 0
            ListBox1.Clear
            
        Case 1
            'Redefine list for team members Team 1
            myArray = Split("Select Team Members Team 1|Dave|Rob|Sarah|Dave|Rob|Sarah|Liz|Mike", "|")
            ListBox1.List = myArray
        Case 2
            'Redefine list for team members Team 2
            myArray = Split("Select Team Members Team 2|Mike|June|Mary|John|Steve|Maria|Liz|Andy", "|")
            ListBox1.List = myArray
            
        Case 3
            'Redefine list for team members Team 3
            myArray = Split("Select Team Members Team 3|Steve|John|Mary|Ivan|Dan|Lisa|Ian|Joan", "|")
            ListBox1.List = myArray
            
    End Select
    
End Sub

Private Sub ListBox1_Change()
    Dim lngIndex    As Long, lngCOunt As Long
    Dim arrTMs()    As String
    lngCOunt = 0
    For lngIndex = 1 To ListBox1.ListCount - 1
        If ListBox1.Selected(lngIndex) Then
            ReDim Preserve arrTMs(lngCOunt)
            lngCOunt = lngCOunt + 1
            arrTMs(UBound(arrTMs)) = ListBox1.List(lngIndex)
        End If
    Next lngIndex
    If IsArray(arrTMs) Then TextBox1 = fcnArrayToCommaAndDelimtedList(arrTMs)
End Sub

'**** Create a Comma/And delimited list.
Public Function fcnArrayToCommaAndDelimtedList(varIn As Variant, Optional bOxford As Boolean = False) As String
    Dim strTemp     As String
    Dim lngIndex    As Long
    On Error GoTo lbl_Exit
    Select Case UBound(varIn)
        Case 0: fcnArrayToCommaAndDelimtedList = varIn(0)
        Case 1: fcnArrayToCommaAndDelimtedList = varIn(0) & " And " & varIn(1)
        Case Else
            fcnArrayToCommaAndDelimtedList = varIn(0)
            lngIndex = 1
            Do While lngIndex < UBound(varIn)
                fcnArrayToCommaAndDelimtedList = fcnArrayToCommaAndDelimtedList & ", " & varIn(lngIndex)
                lngIndex = lngIndex + 1
            Loop
            If bOxford Then
                fcnArrayToCommaAndDelimtedList = fcnArrayToCommaAndDelimtedList & ", And " & varIn(lngIndex)
            Else
                fcnArrayToCommaAndDelimtedList = fcnArrayToCommaAndDelimtedList & " And " & varIn(lngIndex)
            End If
    End Select
lbl_Exit:
    Exit Function
End Function

Private Sub UserForm_Initialize()
    Dim myArray()   As String
    'Create list of teams
    myArray = Split("- Select -|Team 1|Team 2|Team 3", "|")
    'Use List method to populate ComboBox
    ComboBox1.List = myArray
    ComboBox1.ListIndex = 0
lbl_Exit:
    Exit Sub
End Sub

Private Sub FillBM(strbmName As String, strValue As String)
    'Graham Mayor - http://www.gmayor.com
    Dim oRng        As Range
    With ActiveDocument
        On Error GoTo lbl_Exit
        If .Bookmarks.Exists(strbmName) = True Then
            Set oRng = .Bookmarks(strbmName).Range
            oRng.Text = strValue
            oRng.Bookmarks.Add strbmName
        End If
    End With
lbl_Exit:
    Set oRng = Nothing
    Exit Sub
End Sub