Chicken and the egg. If you're sufficiently close to people to convince them to use SMS/Signal/Matrix/whatever with you, then sure, you'll be fine. But what if you're not? What if it's either "install WhatsApp and join our group chat" or nothing?
The secret is to focus on quality, not quantity, and realize a lot of FOMO is involved.
For example, if they write - in the group - that the next meeting is 3 PM next Wednesday at so-and-so, unless you happen to run into them at that specific time your chances of knowing this are slim.