Every Fleep conversation has a unique conversation link (URL). This link can be used to let people join the conversation, if the "Anyone with the conversation link can join" conversation setting is enabled. Additionally, members of the conversation can use the link to refer to the conversation, as clicking on the URL will open it for you in Fleep.
How does it work?
If you want to invite people to the conversation via the link, you have to enable the "Anyone with the conversation link can join" conversation setting. Then, anyone with the link can use it to join the conversation. If they are not Fleep users yet, the will be prompted to sign up and will then be able to join.
Here, the setting has not been enabled yet:
If you have enabled joining with the conversation link, you may also want to make sure the "Show previous messages to new members" setting is enabled. This will make sure the new people joining will see the message history in that conversation when they join.