Assault battles and territory battles are two different things.
Hoth Rebel Assault is a territory battle, a guild event.
How to deal with it varies from guild to guild, and is usually coordinated by 1-2 officers. If your guild is not coordinating these events, find a better one, you'll get more rewards than in a guild that has no directed approach or instructions for guild members.
You have three things to do as a guild member: platoons, missions, and deploy.
We approach a territory battle (TB from now on) by first filling all the platoons that we are able to. We use a Discord bot to assign them, so that the weaker versions of a character are platooned, and players with stronger characters keep them for offence. A well organised guild will know what platoons they can and can't fill, even without bot support.
Then we do all the combat and special missions. When you select a team for a mission, they are automatically deployed to that territory. Check all the missions before starting, as some require specific toons or factions to complete them, and you could leave yourself without a prerequisite team if you just randomly start battling.
Finally, there is deploying. This assigns toons that haven't been used for platoons or missions to a territory, and their GP is added to the score for a territory. Officers should advise which territory(s) to deploy too. Sometimes your GP could help earn an extra star, and placing your characters in the wrong place could result in poorer rewards for the whole guild.
Your roster refreshes at the end of each phase (phases are 24 hours long for the Hoth TBs, 36 hours long for the Geonosian TBs). At the start of a new phase, you have your whole roster available again. If you earned a star in a territory during the previous phase, you shift to the next column. If you don't, you stay in that territory, but all progress is saved and can be built upon in the current phase.