Hello TJ Comerford,
Many thanks for the prompt response.
To answer your query, one fact is you need admin credentials to access the admin center and add these users. If your former IT professional passed these on to you, there shouldn't be a problem adding users yourself.
For the second query, can you kindly clarify on this? By groups server do you mean email host or domain host? If its domain host and this domain is connected to Microsoft 365, this means that Microsoft is your Mail provider and there should be no issues adding the users (email addresses).
If you need any further clarification or incase I skipped something, please let me know.
Looking forward to your response.
Best Regards,
Ashraf