If the threat model for the communication within your group is concerned with attack or coercion from hostile governments, then you have the ability to set up your own email server and convince your participants to switch to it. It's not really much to ask -- it's fairly routine for companies and organizations to issue email accounts to employees/members and require their use for official business.
Consider that it may be easier to ask your communication partners to start using a new email account you've provided them than to adopt a new communication platform.
On the other hand, the people you're communicating with have chosen to trust the organizations that they're using to provide them with those services. Whatever service they're using is not radically different to trust than it is to trust the Signal developers, or the Google Play Store or Apple App Store to distribute the Signal application to your mobile device, and so on. Even if your participants are using Signal to communicate, they may have online backups enabled with their device to Apple, Google, etc., such that without your awareness your communication partners are trusting them just as much as if they were email providers.
