To commit oneself, as to a project, a goal, an organization, a cause.
To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours.
To join something, after signing.
To apply to receive unemployment benefits.
To log on; to start using a computer, radio, etc., or to start talking.
The time of day when a radio or television station begins broadcasting, usually after being off the air for several hours.
To add a name to the list of people who are participating in something.
To add one's own name to the list of people who are participating in something (including amateur, but not professional, teams)
To agree to purchase some good or service.