sign up

verbs

engage by written agreement
"They signed two new pitchers for the next season"
contract, sign, sign on, sign up
join a club, an activity, etc. with the intention to join or participate, "Sign up for yoga classes"
sign up
is a type of (narrower)
become part of; become a member of a group or organization
"He joined the Communist Party as a young man"
fall in, get together, join