I have two friends, one recently purchased Shadowlands (brand new account) it says I can’t add him to the guild because he’s a free trial, but he paid for a subscription? The other friend last played in WoD and resubbed, when I try to add him it says “This account cannot join guilds without an exisiting character in the guild”
Why are these accounts restricted to joining a guild?
Those are what we refer to as “starter” restrictions. Give it a little time (shouldn’t be longer than 24 hours) and those restrictions will lift on their own. Then your friends will be able to join.
The system is basically just watching to see that the game time payment goes through successfully.
Starter Edition accounts are often used to spam illicit services and gold sales, etc… If one of these accounts proves not to be a legitimate used and manages to get invited to a guild they could cause some issues for the guild that inviting them ( spam, advertisement, etc…).
The restrictions are just a means to limit the impact those folks have. As Perl mentioned the social restrictions should lift within 24 hours, often way less. 
Thank you both!