There are two ways to communicate with ppl outside your org without federation (this is covered on the website):

1) Set up a second TMTP service where customers and/or suppliers can create accounts, along with employees who need to interact with them.

2) Have some employees join a third-party service which is open to all involved in your field. There is a risk of phishing in this case, but:

. a) anyone you haven't previously contacted is limited to short, plain-text communications to you (see Ping in protocol), and

. b) such third-party services would typically charge a fee to members and impose a small cost per-ping, and

. c) you know that you're dealing with unknown entities with possibly malicious intent.

TMTP clients support active logins to accounts on any number of TMTP servers, just as browsers support multiple active connections to websites.