With Yapla it is possible to add an unlimited amount of users. However, users must have a license available at the time of login.
A license is like a token, when a user logs in, he uses a token (license) and when he logs out, he releases a token (license). Depending on your usage, you need more or fewer licenses.
For an organization where each resource spends a good part of the day on Yapla, it is better to have one license per user. For an organization with volunteers, where people only log in a few times a month in the evening, the number of required licenses is much lower.