You cannot summon a boss at someone else's Garrison. Only that person can use their item to summon. Make sure the bosses are summoned by the leader of the group, whose garrison you are at.
Please see our full Garrison Invasion Guide for more information on invasions. This guide will list each bosses' abilities and give a brief overview to the fights. These bosses are meant for 10-40 players to defeat, and don't have too many abilities so should be easily defeated by a coordinated raid. Completing a garrison invasion has a chance to drop an item that can summon a raid boss to your garrison.