Signing up
Register at entrancegrp.com/register. You do not need a finished customer list on the first day. Signing up does not send a campaign.
A few things are worth doing before the first send.
Registration
Regular sending waits on an approved 10DLC campaign. The form checks the website you enter, how people opt in, and whether the sample messages match that use. Registration is described here. The 10DLC note explains the terms the form uses.
Numbers
Numbers are ordered in Manage Numbers at $1 a month. If you are choosing a first area, start with the state you operate in, or the state your customers are in. You can add and remove numbers later.
Billing
The bill is on the first of the month: segments at the tier your volume reaches, the account fee for that tier, and $1 per number. Carrier fees are included in the segment rate. How billing works walks through a month. The pricing page estimates other volumes.
Other people
If someone else will work in the workspace, invite them from the account page. A separate registration creates a separate account, and support has to join those by hand. Adding users is the short version.
When the account, the campaign, and the list are in place, send the first campaign.