Checking Data Against 3 Criteria Then Displaying Results In Combobox?

using a command button to input data back into the spreadsheet at a specific location.

Background: I am building a time clock spreadsheet, of sorts. I have a user form that provides a list of volunteer names in a combobox and then a list of activities they can perform in another combobox. Some activities have Details. (So, VolunteerName Todd can choose Activity Maintenance, which has no details; Volunteer Joe can choose Activity Teaching and then can select Details Intermediate 2.) The volunteer then clicks a SignInCommandButton.

The SignInCommendButton populates a worksheet (VolunteerLog) with the following information:

Column A: Volunteer Name
Column B: Today's Date
Column H: Exact Time In
Column E: Activity
Column F: Details, if populated

The SignInCommandButton also copies a formula from cell L2 into the appropriate row of column C (Time In) that rounds the Exact Time In to the nearest 15 minutes. The SignInCommandButton then does a Copy/Paste Special Values to remove the formula from column C and then resets the Userform.

What I need to do:

1) When the person first selects their name from VolunteerNameComboBox, I want to check 3 things:

a) whether the person's name exists in Column A of the VolunteerLog worksheet;

b) if it does, if the Date associated with that entry = Today; and

c) if it does, if the Time Out column is Blank.

If all three conditions are met, I want the ActivityComboBox to populate with the values in Column E and F, if necessary. The user will then click the SignOutCommandButton. (details in a moment)

I already have the code for if the conditions aren't met (Activity box populates and, depending on the selection in the Activity box, the Details box may appear for a selection to be made, or may remain hidden.)

2) The SignOut Button needs to enter the Time into Column I of the appropriate row found in (1) above. It will also need to copy the rounding formula I mentioned earlier, and do the Paste Special, but I have that already.

For (1), I think I have a start. My thinking is to first check if the Name selected in VolunteerNameComboBox is in the VolunteerLog. Each time it shows up, I'd like to add it to a list (range?) called rngSignedInDate. Then I'd loop through all the entries in that range checking if the Date = Today. If it does, then I'd add it to a list called rngSignedOut and loop through those results to see if Column C (Time Out) is blank.

I think I have the first loop, but am stuck on how to populate the results into rngSignedInDate. Here is the code:

Code:
Private Sub VolunteerNameComboBox_Change()
'Check if the Volunteer has signed in already
Dim strVol As String
Dim rngSignedInVol As Range
Dim rngSignedInDate As Range
Dim rngSignedOut As Range
Dim rngSignedInActivity As Range
Dim rngSignedInMatch As Range

[code]....

(2) I haven't started working on the SignOutCommandButton. The challenge for me with that is directly tied to the challenge in (1). If someone does pass all 3 tests, I need to have that specific instance defined in a way that I can have the SignOutCommandButton put the time in the appropriate row.