If you have an AmongFolk offer code, open You → Plans & billing and choose the group you want to try Plus with. Enter the code in Offer code (optional), then choose Start free trial.
You need to be eligible for a trial and have verified your email. A trial is for one group, not five Together groups. Check the end date shown after the offer is accepted.
No card is needed and the trial does not automatically become a paid subscription. If the code is refused, check the spelling and whether a trial has already been used. Contact support if the offer still looks wrong.
Buying Plus or Together during a free trial starts the paid subscription immediately. Read how the standard trial works.