New accounts get 100 credits once. No card is required. The grant is one per person: creating or joining another team does not add another, and disposable email domains do not qualify.
The grant is pack credits. It does not expire. If the team later has a monthly plan, plan credits are spent first and these stay until they are used.
A device fingerprint hash is kept so the same person cannot claim the grant twice.
Full detail: Credits and pricing and Teams.