When you receive an invite to join a board or have been given a link to join a board you do not need to be a paid subscriber of Ideaflip.
Ideaflip is built for collaboration but it is also secure so you do need to be a free signed up user of Ideaflip but not necessarily a paid subscriber.
When you join an Ideaflip board for the first time you will be asked to sign-up with your email address and name. At this point, you will be asked if you'd like to trial a subscription so that you could experience an unlimited service. You do not need to accept this invitation and can go directly to the board you have been invited to.
If you do take the trial you have unlimited use of Ideaflip for 14 days after which time you will default to a no-plan service. The no-plan service has a limit of 3 new boards and 2 new guests. If you already have more boards and/or guests than this you will not be able to create new boards or add new guests unless you remove existing boards/guests to under the limits.
Remember this limit is for boards and guests that you create/invite yourself, it will not affect your ability to collaborate on the boards you have been invited to by others.