docs: add information about inviting present members (#7490)

* docs: add information about inviting present members

Signed-off-by: Christine <shecoder30@gmail.com>

* Update source/collaborate/invite-people.rst

---------

Signed-off-by: Christine <shecoder30@gmail.com>
Co-authored-by: Carrie Warner (Mattermost) <74422101+cwarnermm@users.noreply.github.com>

source/collaborate/invite-people.rst

@@ -78,5 +78,8 @@ You can share a unique URL that takes users to a Mattermost account creation pag
 
 .. note::
 
-  Users on the same Mattermost server can view the username and full name of other users so they can communicate with them directly, via direct messages, regardless of team membership.
+  Users on the same Mattermost server can view the username and full name of other users so they can communicate with them directly, via direct messages, regardless of team membership. 
 
+.. note::
+  - You can't invite someone to a Mattermost team, as a **member**, if that user is already a member of that team. 
+  - You can't invite someone to a Mattermost team, as a **guest**, if that user already has a Mattermost account. Invite them to a team as a **member** instead.