How do Coalitions work?

A few n00b questions:

1) Are Coalitions mixed tiers? For example, could a bronze tier player join a coalition started by a platinum tier player (or vice versa)?

2) How does one go about finding a coalition that you are even eligible to join. I know I can go the the "Random" tab on the Coalitions page in the app, but that gives you zero information.

I'm silver tier, and pretty new to the game, but I'm good IMHO:
* 333 ribbons in the recent Saheeli event (7th place....sooooooo close to getting Saheeli...)
* 297 ribbons in the Terrors event running simultaneously (Certain secondary objectives were not possible given my card collection--can't cast 2 delirium cards If I don't have any in that color!).

I'd like to be in a decent coalition for tomorrow's event (or later today I guess, since I've slipped past midnight here) but I'm not sure how recruitment works. Point me to where I should be going? (And what I should do once I get there?)

Wink