User Activation refers to the platform sending an introductory "Welcome" message to a (new) user.
In the most common flow, an activation message is sent when a new user added to the platform. You may choose to add users without sending a notification. This allows you to work on the user's profile prior to having them join, and then resend a login prompt.
Strictly speaking, Activation is never required. Once a user has been created, they can login using their registered email — they can attempt to login and specify "Forgot Password", which will send an activation notice to their email.
Our Activation action, available from thesimply sends the same message as the user receives when they select "Forgot Password".
If you want to send your custom Activation message, you can direct users to the URL:
This URL will send users to a page where they can "activate" themselves, causing the custom email invitation to be sent to their registered email. Note, they'll have to be already be added to the system as otherwise the email address won't be recognized.
Use the above URL instead of telling users to find and select the "Forgot Password" link on the site. It's much easier.
A side-effect of activation is we reset the stored password. This means if you send an activation message, the user can only login through the received email message. Their old password is no longer valid.
For sites which use Single Sign-On (SSO) the email is sent, but the user is invited to login. There is no need to set (or reset) a password as the system uses authentication provided by the Identity Provider (IdP).