Hi Kevin,
To do this, you probably want to use a Summarise tool. The Prep work that will be needed is getting a field for each day. The summarise tool will do the min/max, and then a join tool will append to the original data if that was required.
A formula tool with the following will give you the Day field
?DateTimeTrim([Activity Date Stamp],'day')The Summarise tool will just need Group By: UserID, Activity, Day, Min (Activity Date Stamp), Max (Activity Date Stamp)
If you want to append your results back on, then use a Join tool on all the relevant fields to append the Min & Max fields.
Kane