You would want IPS.Connect. It is built into the 3.4.x backend and is designed specifically to support logging in from other applications and installs of IP.Board 3.4.x or above.
It uses a master and slave structure to determine which database will store the real database and which is storing a temporary cache of the real one.
Documentation is found on their website;
http://www.invisionpower.com/support/guides/_/advanced-and-developers/integration/ips-connect-r27
IPS Connect provides a simple solution for any application to to implement single sign-on (SSO) with other IPS Connect compatible applications.
In an IPS Connect network, there will be one "master" application and any number of "slave" applications.
The IPS Community Suite (versions 3.4 and above), can serve as either a master or a slave in an IPS Connect network.
This article will show you how to connect two IPS Community Suite installations using IPS Connect.
Before you set up, you will need to decide which installation will be the master, and which will be the slave.
Generally speaking, when setting up an IPS Connect network, the "new" installation (which does not have any existing user data) should be the slave. If you are connecting two installations which both have existing user data, it doesn't matter which is the master and which is the slave.
While the question is a bit old I hope it can be of assistance to somebody in the future.