Begin typing a name at To:
A box will appear underneath with a suggestion for completing what you type.
You can drag this box down and select a contact.
After the contact fills in on the To line there will be a >,
Tap after the >,
Begin typing a name......
(follow steps above)
I do not know if there is a limit, but I've sent up to 9 individuals this way.
Nexus 5x under Nougat:
Hit + for new message
You get a To: line
Hit All Contacts. It may give you a keyboard but swiping above the keyboard should get rid of that so you can see the entire list.
Find the first one you want and tap Mobile at the right
If it sends you to the typing screen, hit X.
It should take you back to your contacts;
Repeat until all your Tos are on the line.
Click the checkmark
Now type your message and send. It will probably get marked MMS.
I have been trying to get a group defined on Gmail's Contacts (not in Contacts on my Nexus 5) to show up IN Gmail on my Nexus 5 under Nougat, no luck whatsoever. But at least I can do ad hoc multi messaging now.
I have fat fingers so it took me a while to learn to do this reliably.