To enlist (someone) or make (someone) a member of
To enlist oneself (in something) or become a member (of something)
To enter (a name, etc.) in a register, roll or list
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.
To add a name to the list of people who are participating in something.