I addressed this "issue" already via suggestion but I think its good to also do it here.

When I tag "join groups in progress" it is very annoying to get an instant invite (as tank) just to wait for more members in the duty.

I'd suggest to reassemble the group ONLY when all member are available already.
Members already in the duty (the "left-overs") should have the possibility to leave the duty but stay in queue. Or at least see how many members are already in queue for their group.

This is one of the reason I think twice about joining a group in progress specially when I'm doing daylies as DPS. Just as an example, I waited for a group and was doing my leves and got a pop for a group in progress. I canceled my leve-quest (lost my leve of course) and joined a group with only ONE member still "searching for member" we waited for more than 15 minutes and decided to abandon group.

Joining groups in progress has a lot of space of improvement.