Hi @beehive414, welcome to Meteor and these forums.
SAAS with Meteor is completely possible - search these forums for SAAS and you’ll see various threads about it. Separate databases, unique URLs and reverse proxies are one method, but there are others.
In my opinion, the simplest method (as I describe here) is to have one domain name and only one database - this makes devOps much easier (no reverse proxy, trivial SSL certificates, etc.). All you need is a package like mizzao:partitioner to assign a groupId to each record in the database, and then each user can only access data from their own group.
Your use-case sounds similar to mine - an admin would create an account, and then invite people to share documents with. When the admin creates an account you assign them a unique groupId (I just use their own userId), then any users they invite automatically inherit the groupId of the admin. Any other database record these users create inherit the same groupId, and mizzao:partitioner then automatically restricts database operations to records with matching groupIds.
Good luck!