Here's the list of things at issue with WhatsApp:
- WhatsApp guarantees group sizes of 256 to work properly. After that the admins cannot manually add people anymore, however people can still be added via an invite link. However, as WhatsApp supports only groups of up to 256, I feel particularly insecure about going beyond that number. All kinds of issues might happen regarding the quality of the service and the usability of the software, we don't want that to happen.
- Signal supports up to 1000 members in a single group, we should be able to use that for the foreseeable future without any concerns.
- Signal is developed by a foundation, not a for profit company. The software is open source and those programming inclined can review the source code of both the client and server here (if so desired).
- It has a focus on security and privacy.