After you add a new participant, he/she may request for the activation email again OR you may just want to send the email again to a participant after a couple of days or weeks to remind them to register to your site.
All that you need to do, is go to the Participants tab and search for the participant you are looking for in the Search box highlighted below. Then click on the Send Activation Mail link for that participant.
Note - Always remember to ask the participants to check their junk folders in the first instance, if they have not received your email.
The participants who do not have the Send Activation Mail link are already activated, i.e. registered on the site.
If a participant has not activated their account yet and is not receiving the activation email for any reason, you can also ask them to reset their password. If they successfully reset their password, this will also activate their account. This is true for both normally registered participants AND imported participants.