Checking self enrolment is enabled:
In a course, go to Settings > Course administration > Users > Enrolment methods and ensure the "eye" is opened for self enrolment.
Adding an enrolment key to a course:
If you wish only a certain group of users to self enrol (and not anyone) then you can set an enrolment key which you then tell them to use when they access the self-enrolment screen.
Adding a group enrolment key to a course:
If you wish your users to enrol themselves directly into a group in the course then you can set a group enrolment key which you then tell them to use when they access the self-enrolment screen.
Setting the enrolment period:
Tick the "enable" box to the right of Enrolment period and then choose your dates.
Example of sending a welcome message to new users:
http://growdly.freshdesk.com/solution/articles/1000091356-sending-a-welcome-message-to-new