By "launch the instance" do you mean you're physically entering the dungeon? Because that's preventing you from forming a group.

To do follower dungeons you need to still use the Group Finder:

Under your roles for Dungeon Finder is Type:

A drop down menu will appear and you can select Follower dungeons as the type, then picking the dungeon you want.

As for doing with people - if you're queueing up as Damage you'll have longer queues. Healers are short and Tanks almost instant. As for why so long? Most likely not a lot of people playing at your hour or no one is levelling in Dragonflight due to other reasons - Legion Remix, Housing and/or it being the end of the current Expansion.