When you invite people to join your Viima board, it's almost inevitable that some of the users will not respond to your invite, especially if there are many invitees.
There are a couple of things you can do to improve the odds and get them to log in:
Communicate to them prior to sending the invites. The first and most important thing you can do is to simply communicate with your target audience prior to sending the invites so that they know to expect the invites and know why it's important for them to participate. This greatly increases the response rate to your invites.
Re-send the invites. Even if you have communicated well, there will always be people who don't participate, at least on the first invite. In this case, you can always re-invite them. This guide will explain how.
You might also have to re-send the invites if you are using Viima accounts for logging in and the users haven't responded to the invite within seven days, as the invites will expire due to security reasons.