Your first sentence uses the simple future tense (will+present tense) to express a future event or a future state of being while your second sentence uses the future continuous tense (will be + present participle) to emphasise a future action that will occur for some time.
An analogy you can use for comparison would be the difference between simple past tense (e.g. spoke - a past action) and past continuous tense (e.g. was speaking - a past ongoing action).
You will participate (in) the meeting. (Some time in the future, you will take part in the meeting and be completed. Once you have started your participation in the future, you cannot keep on starting to participate.)
You will be participating (in) the meeting. (Perhaps the meeting will take place over three days. You will arrive at the meeting, and is expected to participate continuously throughout.)
You use the future continuous tense often in job descriptions to describe future routine tasks. (e.g. You will be leading a team of dynamic players and ...)
On a separate note, "participate" is very often used with "in".