Hey,
First of all, thanks for such a cool product. Slack is cool, but mattermost let me keep my data on my own server.
Like @sri explained, ldap would be great in the free edition. The ldap filter/search should only be available in the commercial edition.this is the same approach as in gitlab.
This approach would be great because small companies could use mattermost for free. If the company grow (and have more mattermost-users) they need the advanced ldap features or saml. And they don't wanna miss mattermost!
Greetings
Tonka