Do you get an error message when you invite the members or the members get an error message? Could you post a screenshot of it? This way we'll know exactly what is going on on your end and give you the proper advice.
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After clicking to copy the link and getting the "link copied" confirmation, DO NOT USE THE LINK BUTTONS (ie: Messenger, etc) to send it to someone. Instead manually paste the link (ie: CTRL + V) into an email or a Messenger chat. Otherwise the invited member does not get brought to the correct page where they can join the plan.