welcome to the forum.
i don’t believe this is possible in the current system. i don’t think it’s possible to add a list of users in one batch to the project criteria either. i think the way to do this without having to do it all yourself is to set up the new project(s) to include only project members, and then ask the members of the other project to join the new project(s).
i do think it would be useful to be able to set up groups of people so that you could filter that group (in project settings and also as a general filter), but i think that would complicate the way project trust works.
if you don’t really need the project structure, you could set up a project that includes a set of people, and then just filter using that project. so for example, for birds seen by the group, you could filter in the Explore page for taxon_id=3&project_id=your-project; and then for snakes seen by the group you could filter for taxon_id=85553&project_id=your-project. then save / bookmark those Explore URLs somewhere so that you can easily reference them when you need them.