This mod is for Forge 1.15.2
Can be downloaded on CurseForge:
https://www.curseforge.com/minecraft/mc-mods/teams-mod

Team Features

Advancement Sharing (toggleable)

Similar to that of the popular mod "Together Forever"
Should work very consistently, I don't even have a force sync command because it should never be required.
Advancements sync whenever a teammate joins the world, joins the team, gets an advancement, etc.

HUD Overlay (toggleable)

HUD overlay shows you the status (health/hunger) and location of nearby teammates
In addition to being toggleable in the config, you can temporarily hide the HUD by pressing "v" by default

Commands

/team create <teamname> : creates team of "teamname" and automatically adds you to it
/team invite <playername> : invites player "playername" to your team
/team accept : accepts invitation to a team
/team list : lists all teams
/team info <teamname> : lists all players in team "teamname"
/team player <playername> : gets team of player <playername>
/team remove <teamname> : removes the team "teamname"
/teams leave : leaves team
/teams kick <playername> : kicks player <playername> from your team
/teams config <teamname> <configoption> <true/false> : updates config option for team "teamname"
 

Teams GUI

Teams button appears in inventory screen (with toggleable size)

This button opens a menu allowing you to:

Create a team (showing you the taken team names)
Invite a player to your team (showing nearby eligible players)
Kick a player from your team (showing teammates)
Configure your team
Leave your team
List teams and players within them
Set priority players in your HUD
Transfer items to players within your team (in range of 100 blocks or so. No more dropping items on the ground!)
 

Other

Separate chat tab for team to discreetly converse
PvP is disabled for teammates (toggleable)
Sounds are broadcast when your teammate dies (toggleable)

Miscellaneous Features

Player Pinging: When your name is typed in chat, the message becomes bold and you hear a sound (toggleable) 
Team Pinging: When the name of your team is typed in chat, the message becomes bold and you hear a sound (toggleable)
Prefix: When a message is sent from a player in a team, they get a prefix with their team name (toggleable)
Chat Bubbles: When a player sends a message, that message is displayed above their head for 10 seconds