We add our student to groups in a bulk using Conduit. In that case, PLD cannot cope with the number of emails as it happens too fast (100+ new group member in the same second). However...
We still use PLD to send welcome emails. It is a bit tricky but doable.
Go for the recurring event and run it every day. In the condition part, you can check if the student is a member of a certain group and in the action part, you can send the email. But you also have to make sure that you won't send another email on the following day to the same person, so you need an extra group to collect all the students who have been messaged.
- Recurring event: Every 1 days at 12:15PM
- User is member of group: R44-L7MCROL2-7RWM - S1
- and User is not a member of group: !messaged Support
- and User is member of group: 7RWM
- Send email: Welcome to your course
- Add user to group: !messaged Support
In this example above the student needs to be in two groups (R44-L7MCROL2-7RWM - S1 and 7RWM) but not in the group where we collect the 'already messaged' students. We send the welcome emails every day at 12.15pm and add the student to the 'already messaged' group, so next day they will be ignored.
I hope it makes sense.