GarageBand works well for sending your music into a Session. Because it has no buses or aux tracks, recording a Guest in GarageBand is limited.
Open GarageBand > Settings > Audio/MIDI and make sure Enable Audio Units is on. GarageBand won't load the Sessionwire plugins without it.
This is the setup we recommend for GarageBand.
You can put the Sessionwire Receive plugin on an audio track and set HQ Audio Destination to the Sessionwire Receive Plugin. But with no buses, the Receive track feeds the master track, and the Send plugin with it, so your Guest hears themselves come back.

If you do this, only have one of the Send and Receive plugins switched on at a time.
GarageBand also can't record that incoming audio without extra routing through an Aggregate device. To record remote performers, have them record on their side and send you the files, or move up to Logic Pro. See Record someone with the Receive plugin.