My understanding is that if those 50 users already have licenses there is no further licensing requirement but if they do not have licenses you will require to buy licenses. Each user (human being) that accesses a Notes server must have a license.
There are three primary ways to acquire licenses of software in the Lotus Notes and Domino family:
Client-Server - acquisition of individual client and server licenses. This option requires a client license for each user, plus server licenses determined by the number of processor value units* associated with the hardware on which the Lotus Domino server software is deployed.
Per User - acquire based on a flat charge for each individual user. With this model, users have their choice of client license options (including Lotus Notes), and all necessary server entitlements are included. However, access is limited to the specified user community. To extend the reach of applications, utility server licenses can be acquired separately.
Processor value unit* - utility server licenses. This acquisition model is favored by companies who want a cost-effective way to deploy non-mail applications to users inside and outside their organization. Client access licenses are not required for authenticated access by Web browser users.
That makes sense. So we can use a departmental IT Diary based on a Mail-In calendar as we all have a licence for our own email account and don’t need to buy an extra one.