The best way to get designers involved is to ask them the question, "What don't you like about this software? How would YOU change it?"
This shows that you respect their input and are inviting them as a peer to think about the project both visually and on a systems planning level.
I'd also recommend having a design lead who can bring on other designers as needed. The design lead should be responsible for making all design decisions and ensuring consistency. He should also be regularly communicating his decisions and reasoning to the community. (Mark Boulton did great working with the Drupal community, for example.)
When you give feedback to designers, try not to say "Take this out." or "Make this bigger. Or change this color." Tell them why (this is very important) you want something changed, and let the designer come up with a solution. And trust them to make the right one.
Asking them that question also gives you an idea of whether they understand the project and if their ideas for the design align with your own.
But don't ask them to whip together a "quick" mockup, that's considered spec work and is frowned upon in the design community. No design work is both "quick" and good.
This shows that you respect their input and are inviting them as a peer to think about the project both visually and on a systems planning level.
I'd also recommend having a design lead who can bring on other designers as needed. The design lead should be responsible for making all design decisions and ensuring consistency. He should also be regularly communicating his decisions and reasoning to the community. (Mark Boulton did great working with the Drupal community, for example.)
When you give feedback to designers, try not to say "Take this out." or "Make this bigger. Or change this color." Tell them why (this is very important) you want something changed, and let the designer come up with a solution. And trust them to make the right one.