The more people you invite to contribute to the on-line tribute the better.
Not everyone will want to contribute something for the tribute for various reasons. Not everyone will remember or make the time. Some people will send in their tribute too late. And, did you know not all email reach their destinations?
So, some percentage of the people you invite to participate in creating the tribute gift won't for one reason or another.
Therefore, you need to invite as many people as possible.
Also, the more people who contribute, the better the experience will be for everyone. The more tributes there are, the better the chance for humorous ones, touching ones, interesting ones, etc. The more there are, the more special your honored guest will feel.
And you don't want to leave out anyone who would want to write something for the tribute and miss their opportunity. They could still write it after the party, but it won't be as rewarding.
So, for these reasons and more, you'll want to invite as many potential contributors as possible.