When you add users within a billing cycle, the subscription for those new users will be prorated for that billing cycle. Here are some examples that illustrates this.

Example #1:
You signup on Jan 1st.
You add 4 users to your org during the 15 day trial period. Total number of users including you is now 5.
Your subscription goes Live on Jan 16th.
Your credit card is charged $25 ($5 x 5 users) on Feb 16th and every month after that.

Example #2:
You signup on Jan 1st.
You add 4 users to your org during the 15 day trial period. Total number of users including you is now 5.
Your subscription goes Live on Jan 16th.
You add 5 more users on Jan 31st. Total number of users is now 10.
Your credit card is charged $37.50 on Feb 16th.
$12.5 ($5/month x 5 users for 15 days) + $25 ($5/month x 10 users for 15 days)

If you remove users within a billing cycle, the charges for those users will remain for that billing cycle, and removed the following billing cycle. Here is an example that illustrates this:

Example #3:
You signup on Jan 1st.
You add 4 users to your org during the 15 day trial period. Total number of users including you is now 5.
Your subscription goes Live on Jan 16th.
You remove 2 users on Jan 31st.
Your credit card is charged $25 ($5 x 5 users) on Feb 16th.
Your credit card is charged $15 ($5 x 3 users) on Mar 16th.